Implement NAUDIO_02B0.

To implement NAUDIO_02B0, we emulate instructions located at 0x12b0 (inside SETVOL), which
overwrite lowest part of right rate.
Indeed, despite its address 0x02b0 (ie DMEM), SP can only execute in IMEM.
Merge pull request #20 from bsmiles32/naudio_02b0

Implement NAUDIO_02B0.