I've stared at this for hours. I changed soundTick() inside CPULoop(int ticks) of the GBA.cpp contained in the existing plugin to psoundTickfn() which, as far as I can tell, is the new sound tick emulating function of the VBA-M version of Sound.cpp.
Assuming this is even remotely correct, I still have no idea how to replace the SoundDriver functions all over Sound.cpp, for example, inside flush_samples(Multi_Buffer * buffer):