No audio on video files, does not matter player or file type

Macbook pro 13"
OS X 10.5.8
2.53 GHz intel core 2 duo

I was watching some files of tv shows on my computer and the audio stopped working. I was using VLC player, so I switched to Quicktime, and no audio there either. This happened when I tried opening .avi or mpeg4 in either player, and I also tried watching files that worked fine yesterday. Still no audio. Internet sound works fine, all other sounds are working. It is just video files. I restarted the computer and tried updating VLC, but I don't know where else to go from here. Audio preferences all say to use "built in output." Suggestions anyone please?

Do you get the same issue with headphones connected?

Would guess that the audio inside the player is muted - not the main volume control.

If that's not the case, would try an SMC reset - link.