1)What are your frame rates?
My sound was skipping when I had bad frame rates.
2)Are the sound files all there in the ifdg aircraft you are flying? Look in the sound folder inside the ifdg aircraft's folder you are flying. Sometimes these problems happen when you place all the sound files in the main FS9 sound folder instead of the actual aircraft's sound folder you are using.
3)The sound completely cutting out can happen on occasion, but usually restarting the computer solves the problem.
So it could be a matter of putting all your downloading files in their respective folder(s).
I have many ifdg aircraft but the model you are using doesnt matter, its the sound.cfg in the aircraft sound folder that tells FS9 when to use certain sounds, and the directories listed in the sound.cfg(for the ifdg aircraft) tell FS9 to play the sounds listed, so they have to be in the right place in order for the program to find them.
good luck with your problem,