I really like SimCity 3000’s jazz soundtrack. That was the first time I remember playing a game where the soundtrack was released separately.

Honorable mention goes to DX-Ball 2, which introduced me to a lot of electronic music and the concept of module/tracker music. The game by default came with just 4 .mod songs but on the website you could download additional files from the same creator, or add other .mod files from other sources to a folder and they’d appear in the game. First shareware game I ever bought, and paid by sending a money order through the mail because I was too young to have a credit card.