With event logging turned on as well as buttons/switches logging I get:
405875 Button changed: bRef=0, Joy=1 (Q), Btn=2, Pressed
405875 [Buttons] 14=PQ,2,K81,8
405875 SendKeyToFS(00000051=[Q], KEYDOWN) ctr=0
405875 Sending WM_KEYDOWN, Key=81 (Scan code 16), Ctr=1
405984 SendKeyToFS(00000051=[Q], KEYUP) ctr=0
405984 Sending WM_KEYUP, Key=81 (Scan code 16), Ctr=1
406047 Button changed: bRef=0, Joy=1 (Q), Btn=2, Released
406062 [Buttons] 14=PQ,2,K81,8
...but sound doesn't toggle.
If I switch the button action over to a FS 'control' event like you did it toggles correctly (which didn't surprise me at all; all of the other buttons I have mapped are using 'FS control' actions and work perfectly).
This particular button (which is eventually going to be just one of four button actions) is to control the PMDG 737NGX "Fuel Idle/Cutoff" levers. I'll be happy to be wrong, but from the reading I've done I think the only way to control those is to use the PMDG-defined keyboard shortcuts (in this case, ctl-shift-F1, F2, F3, F4 for all four of the necessary actions). That's what started me down this path -- in the past I've only "flown" simpler aircraft that had direct control actions for any buttons I wanted to setup.
I've got a multi-PC setup and I plugged a plain-old joystick into one of the "slave" PCs, configured FSUIPC exactly the same way using a button on that joystick to toggle the parking brakes (no sound on that PC) with CTL-. (period) -- and it worked perfectly. Same version of FSUIPC and of P3D.
I'm kind of stumped.
My FSUPIC.ini file is:
[General]
UpdatedByVersion=4955c
History=GZWLG5SDA0MTUH1JZ4XIN
InitDelayDevicesToo=No
Annotate=Yes
NewInterceptTextMenu=No
UseSystemTime=No
UseMidMouseBtn=Yes
MouseWheelMove=No
MouseWheelTrim=No
MouseWheelTrimSpeed=1
JoystickTimeout=20
PollGFTQ6=Yes
BlankDisplays=No
FixControlAccel=No
FixMachSpeedBug=No
NewDeleteVehiclesForAES=No
AutoScanDevices=Yes
AssignJoystickIDs=Yes
VisibilityOptions=No
OneCloudLayer=No
CloudTurbulence=No
CloudIcing=No
GenerateCirrus=No
SuppressCloudTurbulence=No
MaxIce=-4
MinIce=-4
UpperWindGusts=No
SuppressWindTurbulence=No
SuppressWindVariance=No
WindTurbulence=No
TurbulenceRate=1.0,5.0
TurbulenceDivisor=20,20,40,40
SuppressAllGusts=No
MaxSurfaceWind=0
WindLimitLevel=200
WindDiscardLevel=400
WindAjustAltitude=No
WindAjustAltitudeBy=2000
SmoothBySimTime=No
WindSmoothing=No
WindSmoothness=2
WindSmoothAirborneOnly=Yes
PressureSmoothness=0
TemperatureSmoothness=0
DisconnTrimForAP=No
ZeroElevForAPAlt=No
ThrottleSyncAll=No
WhiteMessages=No
ShowPMcontrols=No
SpoilerIncrement=512
MagicBattery=No
RudderSpikeRemoval=No
ElevatorSpikeRemoval=No
AileronSpikeRemoval=No
ReversedElevatorTrim=No
ClockSync=No
ClockSyncMins=5
ClearWeatherDynamics=No
OwnWeatherChanges=No
TimeForSelect=4
LoadFlightMenu=No
LoadPlanMenu=No
PauseAfterCrash=No
BrakeReleaseThreshold=75
SaveDataWithFlights=No
ZapSound=firework
ShortAircraftNameOk=Substring
UseProfiles=Yes
EnableMouseLook=No
DelayedMouseLookZoom=No
AxesWrongRange=No
TCASid=Flight
TCASrange=40
AxisCalibration=No
DirectAxesToCalibs=No
ShowMultilineWindow=Yes
SuppressSingleline=No
SuppressMultilineFS=No
AxisIntercepts=No
DontResetAxes=No
InitDelay=0
GetNearestAirports=No
LogOptionProtect=Yes
OOMcheck=Yes
OOMcheckInterval=10
WeatherReadFactor=2
WeatherRewriteSeconds=1
CustomWeatherModify=No
SimConnectStallTime=1
InitialStallTime=10
NormalStallTime=1
LuaRerunDelay=66
Console=Yes
ConsoleWindow=-1123,95,-126,683
LogEvents=Yes
LogButtonsKeys=Yes
FSVersionUsed="Lockheed Martin® Prepar3D® v3",3.3.3.3
SimConnectUsed=3.3.0.0
[JoyNames]
AutoAssignLetters=No
Y=Saitek Pro Flight Yoke
Y.GUID={92343EF0-3542-11E6-8006-444553540000}
Q=Saitek Pro Flight Throttle Quadrant
Q.GUID={92348D10-3542-11E6-8009-444553540000}
R=Saitek Pro Flight Cessna Rudder Pedals
R.GUID={92350240-3542-11E6-800A-444553540000}
T=Saitek Pro Flight Throttle Quadrant
T.GUID={92357770-3542-11E6-800D-444553540000}
0=Saitek Pro Flight Yoke
0.GUID={92343EF0-3542-11E6-8006-444553540000}
1=Saitek Pro Flight Throttle Quadrant
1.GUID={92348D10-3542-11E6-8009-444553540000}
2=Saitek Pro Flight Cessna Rudder Pedals
2.GUID={92350240-3542-11E6-800A-444553540000}
3=Saitek Pro Flight Throttle Quadrant
3.GUID={92357770-3542-11E6-800D-444553540000}
[Axes]
PollInterval=10
RangeRepeatRate=10
0=YX,256,D,1,0,0,0 -{ DIRECT: Aileron }-
1=YY,256,D,2,0,0,0 -{ DIRECT: Elevator }-
2=QX,256,D,9,0,0,0 -{ DIRECT: Throttle1 }-
3=QY,256,D,10,0,0,0 -{ DIRECT: Throttle2 }-
4=QZ,256,D,17,0,0,0 -{ DIRECT: PropPitch1 }-
5=RX,256,D,7,0,0,0 -{ DIRECT: LeftBrake }-
6=RY,256,D,8,0,0,0 -{ DIRECT: RightBrake }-
7=RR,256,D,3,0,0,0 -{ DIRECT: Rudder }-
8=TX,256,D,18,0,0,0 -{ DIRECT: PropPitch2 }-
9=TY,256,D,13,0,0,0 -{ DIRECT: Mixture1 }-
10=TZ,256,D,14,0,0,0 -{ DIRECT: Mixture2 }-
[Buttons]
PollInterval=25
ButtonRepeat=20,10
1=RY,38,C65671,0 -{PAN_LEFT}-
2=RY,32,C65734,0 -{PAN_UP}-
3=RY,34,C65672,0 -{PAN_RIGHT}-
4=RY,36,C65735,0 -{PAN_DOWN}-
5=PY,0,C1001,0 -{PTT Transmit On (SB3,RW,AVC)}-
6=UY,0,C1002,0 -{PTT Transmit Off (SB3,RW,AVC)}-
7=RY,2,C65607,0 -{ELEV_TRIM_DN}-
8=RY,3,C65615,0 -{ELEV_TRIM_UP}-
9=RY,7,C66279,0 -{RUDDER_TRIM_RIGHT}-
10=RY,6,C66278,0 -{RUDDER_TRIM_LEFT}-
11=PY,1,C65791,0 -{AUTOPILOT_OFF}-
12=PQ,0,C65752,0 -{PARKING_BRAKES}-
13=PQ,1,K190,10 -{Key press: ctl+.>key}-
14=PQ,2,K81,8 -{Key press: Q}-
15=PQ,2,K81,8 -{Key press: Q}-
[AutoSave]
Next=1
Interval=60
Files=10
SaveOnGround=No
AutoSaveEnabled=No
[GPSout]
GPSoutEnabled=No
[GPSout2]
GPSoutEnabled=No
[WideServer]
WideFSenabled=Yes
[Sounds]
Path=C:\Prepar3D v3.3\Sound\
Device1=Primary Sound Driver
Device2=Speakers (High Definition Audio Device)
Device3=Digital Audio (S/PDIF) (High Definition Audio Device)
Device4=SAMSUNG (2- High Definition Audio Device)
Device5=Digital Audio (S/PDIF) (High Definition Audio Device)
[JoystickCalibration]
AllowSuppressForPFCquad=Yes
ExcludeThrottleSet=Yes
ExcludeMixtureSet=Yes
ExcludePropPitchSet=Yes
SepRevsJetsOnly=No
ApplyHeloTrim=No
UseAxisControlsForNRZ=No
FlapsSetControl=0
FlapDetents=No
ReverserControl=66292
Reverser1Control=66422
Reverser2Control=66425
Reverser3Control=66428
Reverser4Control=66431
MaxThrottleForReverser=256
AileronTrimControl=66731
RudderTrimControl=66732
CowlFlaps1Control=66162
CowlFlaps2Control=66163
CowlFlaps3Control=66164
CowlFlaps4Control=66165
SteeringTillerControl=0
MaxSteerSpeed=60
Aileron=-16384,-512,512,16383
Elevator=-16319,-512,512,16160
Rudder=-16384,-512,512,16383
LeftBrake=-16384,16383/16
RightBrake=-16384,16383/16
[Profile.PMDG737NGX]
1=PMDG 737-800NGX PMDG House Winglets
[Axes.PMDG737NGX]
RangeRepeatRate=10
0=YX,256,D,1,0,0,0 -{ DIRECT: Aileron }-
1=YY,256,D,2,0,0,0 -{ DIRECT: Elevator }-
2=QX,256,D,22,0,0,0 -{ DIRECT: Spoilers }-
3=QZ,256,D,9,0,0,0 -{ DIRECT: Throttle1 }-
4=RX,256,D,7,0,0,0 -{ DIRECT: LeftBrake }-
5=RY,256,D,8,0,0,0 -{ DIRECT: RightBrake }-
6=RR,256,D,3,0,0,0 -{ DIRECT: Rudder }-
7=TX,256,D,10,0,0,0 -{ DIRECT: Throttle2 }-
8=TY,256,D,13,0,0,0 -{ DIRECT: Mixture1 }-
9=TZ,256,D,23,0,0,0 -{ DIRECT: Flaps }-
[JoystickCalibration.PMDG737NGX]
AllowSuppressForPFCquad=Yes
ExcludeThrottleSet=Yes
ExcludeMixtureSet=Yes
ExcludePropPitchSet=Yes
SepRevsJetsOnly=No
ApplyHeloTrim=No
UseAxisControlsForNRZ=No
FlapsSetControl=0
FlapDetents=No
ReverserControl=66292
Reverser1Control=66422
Reverser2Control=66425
Reverser3Control=66428
Reverser4Control=66431
MaxThrottleForReverser=256
AileronTrimControl=66731
RudderTrimControl=66732
CowlFlaps1Control=66162
CowlFlaps2Control=66163
CowlFlaps3Control=66164
CowlFlaps4Control=66165
SteeringTillerControl=0
MaxSteerSpeed=60
Aileron=-16384,-512,512,16383
Elevator=-16319,-512,512,16160
Rudder=-16384,-512,512,16383
LeftBrake=-16384,16383/16
RightBrake=-16384,16383/16
Throttle1=-16253,-512,512,16383
Throttle2=-16384,-512,512,16256
SyncSlopeThrottle2=31/29,90/89,127/128
Spoilers=-16384,16256/16
Flaps=-16384,16383/16
[Buttons.PMDG737NGX]
0=RQ,8,C65966,0 -{THROTTLE1_DECR}-
1=UQ,8,C65967,0 -{THROTTLE1_CUT}-
2=RT,6,C65971,0 -{THROTTLE2_DECR}-
3=UT,6,C65972,0 -{THROTTLE2_CUT}-
4=PQ,4,K113,11 -{Key press: ctl+shft+F2}-
5=PQ,5,K112,11 -{Key press: ctl+shft+F1}-
6=PT,0,K115,11 -{Key press: ctl+shft+F4}-
7=PT,1,K114,11 -{Key press: ctl+shft+F3}-