I was under the impression that the MIDI DSP commands couldn't be used while DMA-mode PCM audio was playing (I'm not sure where I got that from)... maybe this is true of earlier DSP versions, but both are working perfectly fine at the same time on an SBPro2 (CT1600, DSP version 3.xx).
On the other hand, attempting to use both SBMIDI and Direct Mode PCM freezes my system right after S_Init...
Found by CODOR (vogons):