mva format or m4a format?
m4a = mp4 audio format.
You should not need to convert them nor should you as it will result in loss of quality. You probably need to get the mp4 audio codec for your player to play them.

If the files are mva, erm that is not an audio format afaik. AFAIK mva = Video Card Driver. aka an .MVA file is known as a Video accelerator file.