Been having this issue for a bit. Basically the sound stops playing in Sonic 1, Sonic 2 and Sonic 4 episode one. The first time it happened I just redownloaded the games and then it was fine but now ...
So, it is my understanding that, while most of the ports of Sonic the Hedgehog 1991 are straight ports, there are a few officially-released versions of the game out there with gameplay differences. ...