Hello David! I apologize if this is a little late.
As others have said, the Sound, EmitSound, and Speak functions work on the Pi version of MMA. These functions, along with Import, can be used to edit and play MIDI tracks in MMA. To route the output of these functions or a MIDI file to a USB audio device, you need to edit the configuration files on the Pi to use your USB device rather than the HDMI port or headphone jack. This article gives a pretty good guide to doing exactly this. Note that you'll probably have to power-cycle your Pi after changing any config files and please back up everything you edit before making changes!