The simplest method I can think of is just to create a little counter using a global variable. Call is "Track" or whatever.
Now you create a function that when called plays the track according to the number of the counter variable.
Track = 1 -> Play X track
Track = 2 -> Play Y track
You have the "on finished playing" condition, when the music finishes it either randomises the Track variable again or adds 1 to it etc and then calls the function again to play.