On my 5G 30GB iPod, audiohw_close() (which puts the codec chip to sleep) saves about 7.5 mA. More could probably be saved by shutting down some related hardware. Of course, you can only do this when not playing or recording audio. Is this a desirable feature?
If idle power consumption is reduced via this and other means, the end result could be similar to original firmware's sleep mode. You could simply set an extremely long idle power-down time.