From Roll20 Wiki
This video covers how to add and sort music in Roll20.
Go to the Jukebox tab at the top right of Roll20. This accesses the sounds and music library, which utilizes SoundCloud. Click the +Add button, which will bring up a search menu. From there, use the dropdown menu to select if you're looking for background music, sound effects, or to search all of Soundcloud. Enter keywords in the search box to look for sounds.
You can also direct link a particular Soundcloud track by pasting its URL into the search field.
You can Preview a track by pressing the Play button or add them to your current session's library with the + button.
Using Your Own Music/SFX
Roll20’s Jukebox streams audio from SoundCloud. To play your own music or sound effects, you’ll need to create a SoundCloud account and upload your tracks to it. Make sure to set your SoundCloud library to Public which will allow Roll20 to stream your audio.
Adding tags such as “Roll20”, “ambient” or “orchestral” will allow Roll20’s search filter to locate your uploaded audio. It may require up to 15 minutes for Roll20 to locate your freshly uploaded track via a keyword search. Note: Soundcloud’s TOS prohibits the streaming of copyrighted content. Upload at your own risk.
As noted earlier, you can also bypass searching for your audio track on SoundCloud by pasting its URL directly into the Roll20 music search.
Any tracks you've added to your game can be played by simply pressing the play button in the Jukebox tab of the Sidebar. It will be audible to all players in the session. Note that you can play more than one file at a time, and all files automatically loop until paused. Each track has its own volume control which can be used to normalize their volume.
Master Volume Control
In addition to the Game Master's control of individual track volume in the Jukebox tab, each person connected to the game can also use the Master Volume Control located under the Settings tab to control the volume level of all music being played.
Each track has a check box for optionally looping the tune. Before the music is played, uncheck this box to have the music only play once-- otherwise the track will loop as persistent background music until you pause it.
To rename a track, simple click on its title in the Jukebox tab of the Sidebar (after you've added it to the Jukebox playlist). Type a new name and press Return to save. Tracks will assume a numerical and then alphabetical order, so numbering them as a playlist is a quick way to organize.