I downloaded music from the internet and after unzipping it is shown as xxx.MUS which neither Windows or NWC recognized.
Has someone a suggestion?
As you note, NWC does not recognize a .mus file type.
See also:
FAQ - What file formats are supported in NWC
http://ntworthy.com/composer/faq/32.htm
Whilst it may not be relevant to the questioner's particular needs, this link is helpful
http://www.answerbag.com/q_view/17327
Tony