Many thanks for your responses! Growing up in the UK (in the pre-internet era), I had no idea how screwed we were by PAL. I have a copy of Sonic Mega Collection for the GameCube, which runs at 60Hz, so is noticeably faster. For the longest time I thought that Nintendo had cast some magic spell on the old games to make them run faster. Learning that, actually, I was finally getting to play at the same speed as the rest of you (some 20 years later) is pretty funny/tragic.
However, something I missed from my previous post is that when I connect my Mega Drive directly to my TV (via mini SCART), I don’t have this problem with my PAL cartridges (I’ve not tested this since I received my EverDrive, so can’t confirm the same is true for NSTC or PAL ROMs). For example, when I play my PAL Decap Attack cartridge direct on the TV, the music is faster than if I play the same cartridge via the OSSC. This is why I thought that the OSSC may have some setting or other I could fiddle with to address the music…
Thanks again for your input.