Assuming that it has not happened due to effect of some virus, you can do two things.
Rightclick one of the mp3, select Open with >> Choose Program,
Select the player with which you play your mp3s and tick "Always use the selected program to open this type of file" and click OK.
Or if they have changed the extensions as well then keep all the mp3s and avi separately in one folder.
Go to the command prompt,
1. Type d: (Assuming D as the drive of the songs)
2. Type CD songs(assuming songs is the song folder)
3. Type ren *.* *.mp3 and press enter
4. Do the same for the avi files, type *.* *.avi and press enter