Have you tried reinstalling/updating your sound drivers.
I know I had numerous sound problems with freelancer because the drivers were not installed properly and were old.
The sound problems ceasd after I updated and reinstalled the sound drivers.
Also try going to RUN from the start menu and typing in "dxdiag". This will open the directx diagnostic tool. Then go to sound 1, sound 2 and music and do all of the tests there.
Also it is highly advisable to download directx 9b or 9 (9b is a very slightly improved version of 9).
Another thing to do is go to control panel and find out (through system what devices you have then download all the latest drivers for each of them. This should improve the sytem in general and reduce thre risk of crashing.
Basically, i think if your sound acceleration is lower or higher than your hardware acceleration than the sound will mess up, i think.
So do the above and see if that works
.
Make sure your system is free of viruses aswell by scanning using something like macafee, AVG or norton antivirus (AVG is free!).
Hope all of that helps
Oo|GHOST|oO