Check your D2 folder for the d2music.mpq and d2xmusic.mpq files - if they aren't present, copy them into the folder from your disc.
Now, the only time the disc should start spinning is when you first load the game and when the act movies play. :thumbsup:
Been awhile since I last had to do this - one of those two files may all ready be present in your folder, so only copy one or both if they are missing.