Thanks for the help!
I've now installed Perian, but it didn't help. Still no sound. cmd-i in Quicktime reveals these are just plain H264 files. No real info on the audio codec though.

I've also installed Mplayer, which came with it's own set of FFMpeg codecs. The files play fine through Mplayer, but still no sound with Quicktime.

BTW, the files cannot be read by iDVD as well, sadly. frown Could this be related with the sound issue in Quicktime?
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Riocar 80gig S/N : 010101580 red
Riocar 80gig (010102106) - backup