I’m not super familiar with the Genesis, so maybe take this all with a grain of salt.
My understanding is that the PAL version of Sonic the Hedgehog simply runs slower, both gameplay and music, to meet the 50Hz framerate, and that only the 60Hz NTSC versions run at the correct speed. If you’ve only ever played the PAL version, try the NTSC version on NTSC hardware; it plays much faster.
For Atomic Runner, it does sound like you have the NTSC version of the ROM on your EverDrive, and the music playback might be synchronized to the framerate, so it’s playing music at 50Hz that’s supposed to be played back at 60Hz. (Keying cadence of things to framerate wasn’t an uncommon practice; Konami did it for the note speeds in their Bemani games.)
I’m not sure about Decap Attack. If the gameplay and music speed are identical, and you can’t identify the ROM’s region, then my assumption is you have the PAL ROM. If you can confirm it’s the NTSC ROM, then I think it’s possible that game was written to run on both NTSC and PAL, so the gameplay will be at the same speed, but the music playback wasn’t adjusted (like my assumption of Atomic Runner).
If it’s feasible, my recommendation would be to pick up an NTSC MegaDrive/Genesis and prioritize NTSC versions of the games, while keeping your PAL console for PAL-only releases. PAL ports were not always given the love they needed to play correctly, and the MegaDrive/Genesis, like the SNES and PlayStation, can’t correctly play both region games at the correct speed without some sort of hardware modification (Super CIC and dual frequency oscillator respectively). (The NES would need something radical, like a board with both NTSC and PAL chips on it and the ability to switch between the two depending on the game being loaded.) And, whatever you do, don’t 60Hz mod your PAL console, because it still won’t run NTSC games at the correct speed.