i found this in another forum and it fixed mine
OK, I think I might have a solution, as this was happening to me.
• Open "Audio MIDI Setup" under "Utilities"
• Under "Properties For" select the appropriate "Output" (Built In, Line Out, Etc)
• Make Sure "Format" is "44100.0 Hz"
Some applications erroneously change this setting causing some audio to cease to play.
See:
http://docs.info.apple.com/article.html?artnum=300832