So you are saying if you put just the cartridge in it will play the standard music in game, but if you put the soundtrack CD in the Sega CD with the cartridge in it will play the enhanced music instead of the music from the cartridge? I didn't even know that was possible.
Yes, exactly. The game will detect the Sega-CD and check the disc for the alternative cd soundtrack.
Sega never utilized the full potential of the SegaCD. Even 32x games could do the same. Technically, the 32x with its 3D graphics overlay, coupled with the SegaCD, could have provided a full 32-bit console experience similar to the Saturn/PSX. Although, always having to use a cartridge and CD for every game would have been irritating.
However, even just selling Genesis/MegaDrive games with an optional CD soundtrack that could be used for in-game audio or for just listening purposes, would have been a brilliant idea.