See if mediainfo can work out what format it is…
http://mediaarea.net/en/MediaInfo/Download/Mac_OS
The non-app store version is ugly but it can give you the info for free, or get the app store version.
https://itunes.apple.com/app/mediainfo/id510620098
Post screenshots of the results (cmd+shift+4, tap spacebar, click on the window - upload the image from the Desktop).
Quicktime 7 may play it, but the .av extension seems like it is a weird container format?
QuickTime Player 7 for Mac OS X v10.6.3 or later
It may also be worth using the unix file command to see if it can tell us anything useful…
Open Terminal (in /Applications/Utilities/) type the next line
file
(there is a space after 'file')
Drag & drop the desired file into the window & hit return.
Tell us what the result says.