All tracks in asset packs play all the time as it is not currently possible to limit when they play, so themed packs are available for some control via enabling or disabling them ingame. For example you might usually play with only the Overworld pack, but when you're building a nether road you could enable the Nether pack as well.
Pack list:
- Volume Beta (30) - The entire album, including Nether, End, creative and unused tracks
- Simply Survival (6) - Only the tracks you'd be hearing if all you did in vanilla early release is boot up the game and play survival in the Overworld. No creative mode tracks, no album-only tracks, no Nether or End tracks. Keeps the main menu tracks, since otherwise you'd be down to 2 (both of the console exclusive songs)
- Overworld (12) - The Simply Survival track list plus the creative mode tracks (which will still play in survival)
- Nether tracks (4)
- End tracks (2)
- Extras (3) - The album-exclusive tracks
https://drive.google.com/drive/folders/1rHmvEVKWH0Xbzj3recGEImfSrVB1s7X-
Versions include 7.3, the 7.4 nightlies and 8.0+. 7.4 and above display music toasts, and 8.0 and above can be enabled by setting the soundtrack toggle in the audio settings to Mixed (the default setting) or Minecraft (only the base soundtrack)
Future plans amount to setting these up so they only play when and where they should, if that ever becomes possible for music packs to do
Also see The Aquatic Singles and Volume Alpha - Extras