The Sega CD adds two pieces of RAM: 256KB for exclusive use by the sub-CPU (though part of it is eaten by the BIOS) and 512KB for use between the sub-CPU and the main-CPU. The former is called PRG-RAM, the latter is called Work RAM.
Moreover, the latter can either be given exclusively to one CPU, or split in half and give each part to each CPU. Considering how a Sonic game would use it, I'd say that giving all the RAM to one CPU would make the most sense (the other mode is mainly meant for FMV, acting mainly as a double buffer - in fact things like rotation, scaling and such can't be done in that mode).
Doesn't really explain why Chaotix also went with it, since I'm pretty sure I heard Z80 use in the music (the Z80's where the square wave/noise channels come from, correct?), but eh.
They just went with that because they had the FM channels free now, nothing else really. And as has been said, the Z80 doesn't output any sound, it's just another processor.