Hi   !
First, would you check this : Type in the address bar about:config (promise to be careful, if asked) Tye and look for the preference : media.autoplay.enabled and make sure that it's value is set to false
Maybe you'd like to give this add-on a try :
https://addons.mozilla.org/en-US/firefox/addon/flashstopper/
Any good  ?

It may be coming from the current web page.

Happy112 - I did what you said to do. I changed the about:config to false and I also installed the add-on. The music stopped. But now that I have gone back into Firefox, it's started again. It doesn't do this in Internet Explorer or Edge. Should I uninstall and re-install firefox?

There might be something in the sessionstore.js or possibly on the about:newtab page that causes this music to play. Keep in mind that not all web pages that include media files work properly with media.autoplay.enabled set to false.

You can check for problems with the sessionstore.js file in the profile folder that stores session data. When sessionstore.js is deleted you lose pinned tabs and open tabs and you will have to recreate them, so if necessary make a note or bookmark them.

You can use the button on the "Help -> Troubleshooting Information" (about:support) page to go to the current Firefox profile folder or use the about:profiles page.

• Help -> Troubleshooting Information -> Profile Directory:
  Windows: Show Folder; Linux: Open Directory; Mac: Show in Finder
• http://kb.mozillazine.org/Profile_folder_-_Firefox

Would you try this :
3-bar menu => Add-ons => Plugins and make sure that Shockwave Flash is set to 'Ask to activate'

And also :
In 'about:config' type and look for the preference : plugins.click_to_play and set it's value to true
Any difference  ?

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Go to that message, and use the Undo link to its right.

Complete stab in the dark : If maybe you are using Amazon Echo :   make sure you turn off 'Autoplay'.