I don't see any options to disable or enable it. But I have no in-game music or heartbeat sound. I only have the sound effects of spells, pings, and peeps dying, etc.
It is because the game is the PC version and not the Amiga version and it was this version that had the heartbeat and music.
I wish there was some way GOG could add the Amiga version to their library, for me it is the superior version although I know there are some who disagree. Hence it would be nice to have the choice which one to play :)