Quicktime - No Sound With Mpg Files


For some reason, yesterday, when I play a mpg file in quicktime, it plays without music. Mov, mp4 files work perfectly, which makes it all a little confusing. Similarly, the mpg files don't work when I run them from iTunes. Those very same mpg files worked perfectly before so I don't think the problem is with the files themselves. I have tried dumping the preference files to no avail. This seems to have happened after I used a program called "Line In" and when I also used the Audio Midi utility. I have been importing music from casettes using the line in on my iMac. There seems to have been some correlation between doing that and causing the sound problem.

I'm using QT 7.02 and iTunes 5.0.

Any help would be appreciated.