Sounds weird. The MyMusix website seems to be up and works for me. Here's a snippet from their FAQ:

Why I'm not able to play WMA songs on the player?
The player supports MP3 and WMA file formats. There are 2 possible causes for the player not playing a WMA file.
1) The WMA file is recorded with a bitrate less than or equal to 32kbps,
2) The WMA file has copy-protected enabled (DRM protection) but was not transferred through Windows Media Player. The file types (WMA, MP3) will scroll at the end of the song title on the screen when being played.

If you're good on these two points, try emailing their support at support@mymusix.com. Let us know what you find out!