The files are still mp3's. When you installed Nero it just made itself the default player for that type of file and probably for any other file type it can play.
To change them back, just open the folder with the files in and right click one of the files > properties > Change > select player you want > OK. That'll work for any file you want to change. You could've changed them back without uninstalling Nero.
See:
Setting Default video app