I had the same problem. My solution worked for me If you want to try it. Go to your Flight folder and pull up the Soundlist. My last two sections look like this.
[A1Engine]
Priority=LOW
3DSound=TRUE
DopplerEffect=TRUE
Looped=TRUE
NumBuffers=2
FrequencyControl=TRUE
MaxDist=250.000000
MinDist=50.000000
InsideConeAngle=360
OutsideConeAngle=360
ConeOutsideVolume=0
[PropLoop2]
Priority=HIGH
3DSound=TRUE
DopplerEffect=TRUE
Looped=TRUE
NumBuffers=2
FrequencyControl=TRUE
MaxDist=1000.000000
MinDist=50.000000
InsideConeAngle=360
OutsideConeAngle=360
ConeOutsideVolume
Next copy and paste the PropLoop2 and if you do not have it already the A1Engine sound files into your Sound folder. Now go back to the Soundlist and make sure that the new sound files are found.
SoundFile027=A1Engine
SoundFile028=PropLoop2
I use the original sound list and add to it everytime myself, that way I know if I screw up where to look. The secert is to make sure each sound file has a number. I used the original sound file from the SkyRaider and not PropLoop1. My solution works with both prop planes with no problems.