Mega Drive Sound Issue

I'm hoping that someone can help me with a peculiar issue.

I have a Model 1 Mega Drive, which has a peculiar issue.

It is a 1992 build, with a motherboard revision of VA4, so anyone who knows their stuff knows that it is one of the consoles that has "high definition graphics" written above the cartridge port, as well as the lack of TMSS.

For the most part, it works, and it runs games, however, I only have the Sonic games to test (just briefly, I used to have a lot more in the way of games, but I lent my old megadrive/cd out to someone who then moved up north and "couldn't remember where he left it" :suspicion:, anyway since then, I've not really bothered with older consoles, but for some reason, now, I am again).

I have created examples (recorded from the consoles headphone port) that you can hear with these links:

Sonic 1

Sonic 3 & Knuckles (lock on cartridge)

Sonic Spinball

Aside from that, Sonic 3 crashes during the title screen, and the other games work, they just have sound issues.

I'm going to be getting into modding and hacking and to learn what is probably a basic life skill, soldering. I've seen people who have replaced the Video chips, or even have custom chips in thier consoles and so I'm thinking, if I were to get hold of a replacement YM2612, would this fix the issue?

It seems to be only the sound at this point, bar Sonic 3 which seems to be some sort of anomaly.

Any help would be appreciated

EDIT:

Got a working controller this morning (the one I had was bust) and am able to test gameplay.
Sonic Spinball, Sonic 3, Sonic 2 and Sonic & Knuckles (plus all variants S3&K, K in Sonic 2) all play perfectly, but give no sound, and if they do, it just beeps and drones of frequencies.

Sonic 2 crashed on the first attempt to get to "Options" but worked on the 2nd attempt, sound test gave no sound at all. Sonic 3 did not crash this time, but it did restart on the save select screen once.

Sonic 1 is the only game that plays and has its sound, the only similarity to the other games is that the "SEGA" audio is glitchy, as can be heard in the links above.

Here is a YT video I put together showing the issues a bit better: http://www.youtube.com/watch?v=rouX6i1Isq4