You should use the File.DirRootExternal and then the path/filename to play the file. File.DirRootExternal set permissions to the SDcard and returns "/mnt/sdcard". Add your path like:
MyAudio.Load(File.DirRootExternal & "/myfiles/audio", "mysound.mp3" )
Hope this helps.
UPDATE: I just tried this myself and I am getting NullPointer errors on the load command. I am looking into this to see if I can find out why. I'll post what I find.
Margret