Just wanted to keep this thread alive. Not sure if replacing sounds is possible, but man, that'd be awesome. Although the siren is pretty classic, no harm having a different ROM with all kinds of updated sounds.
Genesis sounds are all synthetic, so I don't think you could just replace them with an actual audio sample...it would have to resampled down to a tiny amount of data to fit in the ROM, for one thing, but I don't think the game plays 'samples' anyways, more like 'instruments', if you know what I mean. Like the difference between MIDI and mp3s. The game has 'sound language', and the sound engine reads and 'renders' the noise you hear. I think anyways, I don't have any concrete to back this up with or anything.
Reminds me of when my brother and I were kids...we tried replacing the sounds in "Superstar Ice Hockey" on the Amiga. We could open the sounds in some weird format in Audio Master III, and play them, but replacing them didn't seem to work. We wanted to change the slap shot sound (a tiny 'click') to a shotgun sound. LOL.