@mutantx, sorry for the late reply, but I was on holidays
I've no experience with this sketch code, nor the hardware build of this mp3 player.
Try to contact the author of this player or you can check the general documentation about the use of V_VARx / S_CUSTOM : https://www.mysensors.org/download/serial_api_20
Try to break up things in little pieces when accumulating knowledge about mysensors.
Big complicated sketches can be overwhelming.
I start always, when troubleshouting, with the cleanest version of the script, with minimum code required for operation. And then I build it up.