Which version of Flash?
The first applications I would suspect to intercept would be Anti-Virus/Malware/Spyware. If those could be temporarily disabled to see if the result is the same that'd help to know.
Otherwise do you have any software that rips/encodes/plays advanced media like Nero, FFDShow, CyberlinkDVD, VLC, etc. They sometimes install libraries that intercept video and audio streams to help play them and can sometimes interfere, although this is very uncommon.
Lastly I would try converting your audio source file. What format is the audio in currently? Sometimes just opening it and re-saving it to a new file can fix the issue.
Flash is notorious for import issues of all sorts. You need to be sure your assets are Flash-friendly.
