In terms of sound you had the Genesis hardware plus PWM... PWM could have been really useful for some things with the DMA feature it has, but it wasn't documented for developers to use for some unknown reason. Though Chilly Willy got it working quite well:
http://gendev.spritesmind.net/forum/...c80b40dce24b69
With DMA you have WAY less CPU overhead to deal with and could easily have something on par or better than the NDS's sound system. (DS is 8 channels up to 32 kHz with stereo PWM output at 9-bit resolution, 32x resolution/max sample rate are variable, but at 9-bit resolution you could have 44.1 kHz) The PWM is stereo and variable resolution/sample rate as mentioned, see:
http://sega-16.com/forum/showthread....090#post145090
It's only 1 channel with stereo output (also a mono mode), the rest is up to software mixing, but with DMA and the resource in the 32x you can mange a lot. (chilly willy's sample synthesizer being worked on is 32 channels using DMA and mixing with the slave SH2)
PWM also has artifacting, especially at low sample rates (squealing sound), resistor DACs as used in the SNES, Sega CD, etc (most things -and onboard the Genesis), you didn't have that problem.
However, without DMA (as all developers were stuck), you could only do software/interrupt driven PWM playback, so tons of CPU resource, and you'd be lucky to manage 8 22 kHz channels with the slave SH2. (more likely 4, unless you dropped to lower sample rate too -and PWM sounds particularly bas as sample rate drops) For that reason, it was often worse off than the SNES... plus if you had to use the slave SH2 for other things, you were even worse off. Actually, due to the PWM as it was, it wasn't really better than what the Genesis already had with the bare 8-bit resistor DAC (no PWM artifacting ;)) except it had stereo and it didn't disable the 6th FM channel on the YM2612. (otherwise, without DMA, it was only better because of the added resource)