First, go ahead and submit a Support Email Ticket. Since you just got the unit and have only had it for a few days.
Run through this link of changes that must be done for Windows 7. I am sure They did before sending it, but just to make sure.
http://www.mtu.com/upgrades/hoster/windows7-changes.htm
I am wondering though what is the source of the MP3 you are trying to play? Was it imported through Windows Media Player or imported on another computer and transferred?