It's entirely possible to replace sound effects, voices, and musical instruments in old Data/East Sega games. However, the musical sequences themselves are in an unknown format. Most sounds are 8-bit 8KHz PCM. A select few are encoded in a proprietary ADPCM format that has not been reverse-engineered. Newer Segas use 12KHz sounds. Stern Whitestar games use 22KHz VOX ADPCM. SAM games use 24KHz ADPCM (probably VOX as the codec is known).
Starting with SAM Stern started using pre-rendered music tracks in their newer games. Instead of using chip music, it's now a single track. Which makes them a cakewalk to replace. Do Data East games really use sequences in MIDI format? Or was that just an analogy?
To compare, Jersey Jack uses straight-up 16-bit 44KHz stereo PCM. Full CD quality. We should've seen this a good decade to 15 years ago.