I need help changing files of the .mus format into .ogg or.mp3 files; I use Widows Vista.
.MUS files may be Finale files, in which case you will need an appropriate version of Finale to open them. http://www.finalemusic.com
.MUS files may be Minecraft music files. Apparently there is a decoder for these called “musdecoder.jar”
There are also several other (different) formats that a .MUS file could be (search for “.MUS file” on Google).
Audacity does not support either of these file types so you will need to convert them to a supported format before you can use them in Audacity.