Audio stopped working on MacBook Pro. What happened?

Audio playback and scrubbing on the timeline was working fine. Then one day I open the program and it doesn't seem to work anymore. I checked to make sure the audio properties was set to 'Stream'. It was. Other than that everything seems unchanged. I've done everythingI can thinbk of to see what might be wrong, even opened the file on a Windows computer where it worked fine. However, on my Macbook Pro nothing. Anyone else have this problem? Thanks.