"MUS" is a MusicTime Sound file or a Myriad Music file.

There is software for converting .mus files described at
http://www.sibelius.com/.../score_converter.pdf
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"There might be other formats of MUS file, but I've used the following tricks to convert MUS from Finale Notepad 2004 to WAV or MP3...

1. Download Super MP3 Recorder Professional v2.15 from Mp3DO
2. Install Super MP3 Recorder Professional v2.15
3. Register the program by purchasing it.
4. Download Finale Notepad 2004 from Finale Music' and free register.
5. Run Finale Notepad 2004, create song (creates sheet music, multiple instruments)
6. Load MUS song you wish to convert.

Run 'All Sound Recorder XP' v2.15

In 'All Sound Recorder':
Right-click name next to where it says File Name'
Choose 'New File'
Under 'Output Format' choose either WAV or MP3
Change file name to whatever you want it to be called.
Click 'Save Settings' if you want that as the default.
Click 'Apply'.

Click switch to 'On'

Click round Red Record Circle and it will, then, record any sound you hear from your speakers.

Flip to Finale Notepad 2004'
Play Finale Notepad song.
After song is done playing, stop song.

Return to 'All Sound Recorder XP'
Click square STOP button to stop recorder.

Output WAV or MP3 file will be found in C:Program FilesAll Sound Recorder XPoutput folder."

--- http://www.hitsquad.com/smm/wwwboard/messages/157124.html

A simpler solution: Use the appropriate shareware, a single download.

Download "Melody assistant" from Myriad. It's shareware, use not limited in time. As mentioned above MUS files are the Melody (and Harmony by the same company) format.

http://www.myriad-online.com/ /en/products/melody.htm

I use Melody all the time - fabulous. Much greater flexibility than Notepad as far as I'm concerned.
I use it with OMER which is a score recognition tool by Myriad. Though OMER I find more difficult to use, requires a fair bit of tuning to recognize scores properly.

use fanale notepad