Should be a capacitor of some sort in the audio path on the PCB, no ?
You can probably find detailed pictures of various models and their audio circuits on sega-16.com for example, idk if this helps but it has some good info on ithttp://www.sega-16.com/forum/showthread ... io-circuit
The thread is about a replacement audio circuit, but idk maybe it will have some info you can use from the images or other posts.
Different models have different lowpass filters. The model 1 has a much heavier-handed one, but I like it the most because that's what I grew up hearing. Also depending on the model the audio circuit itself could sound worse (substituted / cheaper parts, chips, etc). Generally people distinguish the most between Model 1 & Model 2. Model 2 sounds more "harsh" as it lets more of the highs pass through.
Here's a sound quality threadhttp://www.sega-16.com/forum/showthread ... s-and-more