Hi, Steve

For such model motors it's commonly admitted ~ 3-4000 Hz for ferrite magnets ( to let them some life ...) , and what you want for rare earth magnets.

Note at higher frequencies, it will be difficult to drive the Mosfets directly from the PIC, due to their High input capacitance ...TC 442x drivers are then to be used.

soooo, in all cases, you'll have a sweet music for free...

Alain