I gave it a quick test myself earlier and can confirm it's working in RL 2.7. To add these music tracks to any pack, just edit the levelpack.ini so that the following items are placed in the music selection:
If the levelpack.ini originally had more than 15 tracks, it will be necessary to duplicate part of this list to fill the remaining slots - or, edit the rest of the levelpack.ini so that the levels never refer to more than 0-14 as the music choice - it's the number after the comma for each level entry, e.g:
It may be worth adding the ability to override the default music so that it isn't necessary to edit the levelpack.ini files, which generally shouldn't be edited, particularly if it's someone else's pack. I'll certainly look into this.
Code Select
# music selection
music_0 = CANCANP.MID
music_1 = LEMMIN1P.MID
music_2 = TIM2P.MID
music_3 = LEMMIN2P.MID
music_4 = TIM8P.MID
music_5 = TIM3P.MID
music_6 = TIM5P.MID
music_7 = TIM6P.MID
music_8 = LEMMIN3P.MID
music_9 = TIM7P.MID
music_10 = TIM9P.MID
music_11 = TIM1P.MID
music_12 = TIM4P.MID
music_13 = TENLEMSP.MID
music_14 = MOUNTAIP.MID
If the levelpack.ini originally had more than 15 tracks, it will be necessary to duplicate part of this list to fill the remaining slots - or, edit the rest of the levelpack.ini so that the levels never refer to more than 0-14 as the music choice - it's the number after the comma for each level entry, e.g:
Code Select
fun_0 = 101 Just Dig!.rlv,0 <--- the last number on this line here refers to music track 0
fun_1 = 102 Only floaters can survive this.rlv,1 <--- 1
fun_2 = 103 Tailor made for blockers.rlv,2 <--- and so on...
It may be worth adding the ability to override the default music so that it isn't necessary to edit the levelpack.ini files, which generally shouldn't be edited, particularly if it's someone else's pack. I'll certainly look into this.

Always display a positive number.