Sumi
Yes, there are many free aps that will convert the file to MP3 - https://duckduckgo.com/?q=how+to+convert+aav+file+to+mp3&t=opera&ia=web
inclusive_disjunction
If you meant an AAC file, that can be handled by almost any media converter. Audacity, for example, can import AAC files, and then you can export them as MP3. There are several online converters that can do this if you're one of those people that absolutely refuses to install anything.
https://sourceforge.net/projects/audacity/
http://www.convertfiles.com/
If the file actually does have a .aav file extension, it may be in a proprietary format. The .aav extension is usually used for video, not pure audio, though you can still import videos into Audacity and export the audio tracks.