Yes, DRM'd music can only be played by Windows Media Player. By the way, I've heard that MusicMatch does not support the Axim PDA's.
For an alternative, check out
Yahoo! Music Unlimited. It supports both the Axim X50 and X51 series and music can be saved to a storage card. Many people on Aximsite, including myself, have gotten PlaysForSure DRM'd music from there to work fine. Search the forums for "yahoo music unlimited" for info and reviews. Good luck!