Well, a true AVI file can hardly contain a virus. Of course, it is possible that an false alarm appears simply because of the “random” content of the file - but it’s highly unlikely. It’s a pity that you deleted the file - we may be able to tell you more if we could check it.
Another possibility is that the creator of the AVI file was infected - and piece of the virus somehow got from the memory into (uninitialized areas of) the AVI file. Of course, they couldn’t do any harm this way - but an antivirus may detect them inside.

As I said - AVI files cannot contain a virus, so I don’t think it’s necessary to perform any additional scan.

At what occasion does avast! scan your MP3 files? On-access or on-demain scan? How do you know it checks them?