How do I send a midi command to play a stored mp3 on a pi rasberry?
Also how do I play tracks on a DAW on the pi rasberry using the arduino via usb?
How do I send a midi command to play a stored mp3 on a pi rasberry?
It is not actually part of MIDI as such. You can send a control change message to what ever is receiving MIDI on the Pi. That then will have to be coded to respond to that control change message and play the file.
Also how do I play tracks on a DAW on the pi rasberry using the arduino via usb?
Same thing, but if you are using a DAW you might not be able to program up that interaction into the DAW but you could with a separate python or processing program.