To use a Model 1 Genesis with a Sega CD you can use the mixing cable for better results. Plus, the sound quality would depend on the types of games you're playing. Sega Genesis audio would be processed by the Genesis, and Sega CD audio would be processed by the Sega CD. Without using the mixing cable the sound is all processed through the Genesis. With Model 1 Genesis systems you'll only get stereo sound through the headphone jack, meaning you'll have to use the mixing cable if you want to do what you're suggesting with the Sega CD audio output. With Model 2 Genesis systems, the mixing happens through the expansion port so you don't need to use a mixing cable. Either way, this improves the sound quality with CD audio by not having it processed through the Genesis. Genesis audio is still processed with the Genesis either way, so the quality of the sound chips still matters.