Hmmm, 48 mHZ ARM- sounds like a teeny. If it is, just realize the cortex-M4 does not support double precision in its hardware registers, and you will either have to sacrifice the slow speed of doing double calculations in software for precision, or trade-off massive amounts of round off error with single precision numbers in order to keep the speed up.
There are a lot of good tips on architecture, optimization, and some basics of the ARM CMSIS-DSP library in Yui's definitive guide to the M3 and M4 :
Whatever you're using, it's still probably not the quality of something like the Sharc or Blackfin, so careful with your "real Synth" claims until you have sounds to back it up. 