This mod is made for the RSDKv5U decompilation for x64 Windows. If you want to use it with any other decomp version you will have to build it yourself. Sonic Mania on Steam has its own mod available on GameBanana.
This mod enables SMPS (the sound driver used for the original Mega Drive Sonic games) music to be played in place of the regular OGG files. By default it's set up for Sonic Mania's soundtrack, but you can change the settings by editing the modSettings.ini file in this mod or by making another mod that goes above this mod in the list. I have heard that it will work with older games running in the decomp, but I haven't tested it myself, and you'd need to set that up yourself.
Source Code: https://github.com/MainMemory/RSDKv5SMPS