These files are Melody Assistant .mus files.
If you haven’t already done so I can whole heartedly recommend
downloading and trying out Melody Assistant from www.myriad-online.com

This excellent little program will allow you to write, play and print out music with the resultant files being small enough to email without upsetting anyone.

If you don’t want to download Melody Assistant you can still view and play the files using the smaller (free) Melody Player

Peacocks Collection (zipped file)