If you have a web browser(s) open, then check each browser tab to see if any videos or music is being played or perhaps it is some audio advertisement. Some browsers have an audio or a volume icon on the tab when audio is being played in that tab. Closing the browser(s) is the quickest way to know if this is the source of the music.
Try booting into Safe Mode as this disables third party drivers, startup items and all login items.
You can also create a new user account, then log into the new user account to see if the issue still exists. If the issue is gone, then the issue is within your other user account (most likely a login item).
Have you checked each of the icons on the top right of the menubar to see if any of those items can control the music?
Have you tried to use the media "Play/Pause" button on one of the Function keys or the Touchbar?
You can see all running apps and processes using Activity Monitor located in the Utilities folder. You may need to click on the "View" menu and select "All Processes".