This isn't so much a technical question as more of a plea for clarification. Now on all the various Sega CD FAQs (and a number of posts on these forums) whenever the topic of using a model 1 genesis with a sega cd the mixer port comes up.
The general belief is that the only way to get stereo sound out of this combination is by using the mixer port. From my own limited testing that doesn't appear to be the case.
Now the AV port on the model 1 genesis only produces mono audio, but if you are using a cable with a stereo breakout (such as pcenginesales awesome model 1 Scart cables) then you can use the stereo headphone jack on the front of the genesis.
If nothing is plugged into the mixer port then the sega-cd will transmit its audio signal through the genesis, and you will actually get stereo sega-cd sound via the front headphone jack (in addition to the stereo genesis sound). You can test this by unplugging the mixer port and playing an audio cd on the sega-cd while listening via the headphone jack.
Maybe this is common knowledge but whenever I see people asking about how to setup their model 1 genesis + sega cd this never seems to come up. It seems like this would be a preferred method as it's one less loop the audio has to jump through before getting to your TV.
One capability you do lose by doing this is the ability to independently adjust the genesis volume from the sega-cd volume. If you plug a (non-genesis) source into the mixer port you will notice that the sega-cd quits sending its audio via the genesis and just outputs audio the RCA ports on the back.
So like I said before maybe this is more commonly known than I thought but I wanted to throw this out there.