I know it isn't Arduino, but there is a VERY good SID emulator running on a Parallax Propeller using only one cog (core).
With the 8-core propeller, you could devote 3-4 cores for SID chips (12 voices) and the rest in interfacing with an Arduino.
The same guy has also perfectly emulated the SN76489 and the AY-3-8910 as well. So one $8 propeller (plus an eprom and glue logic) could emulate a SID, SN, AY, etc.