AVI's are video and audio container formats for video & audio streams that are encoded (compressed) using Codecs.
The two streams are independant and interleaved. The fact that they both start at the same time and have the same length alows for synchronization.
If either the audio or the video or both won't play then eiter of the streams may contain rubbish (quite a common 'trick' for those ppl who don't want you to watch thier movie but do want you to waste your time & effort dl'ing it) or your computer does not have the correct CODEC installed which is necessary for your computer to unravel and therefore play back the compressed media.
I suspect the latter.
Installl GSpot AVI on your PC (sounds rude, but it's a must have proggy for all things video on a PC).
Drag & drop movie onto ICON to get an understanding as to the codec its is looking for & post back.
Andy