Sorry, my bad - you already mentioned that it is H.264, about which more here:
http://www.apple.com/quicktime/technologies/h264/faq.html
As you can see these should play in Quicktime 7.5.5 which you presumably have?
As for the missing audio:
Your audio settings may have been changed.
1. Open Audio MIDI Setup (/Applications/Utilities/), then check the Audio Output setting.
2. Change the Audio Output setting to 44100.0 Hz.
3. Quit Audio MIDI Setup.