@leodafox2 At the URL opera://settings/opera-gx, there's a "background music" setting above the "Shaders" setting that takes you to the URL opera://settings/background_music_settings when you click it. On that page, you can enable/disable background music and change the volume etc.
You can also click the Easy-setup button on the right side of the address bar, scroll down and toggle "Background music". There's also a configuration link that takes you to opera://settings/background_music_settings when you click it.
At the URL opera://mods (the Mods sidebar panel), in the circuit diagram at the top, there's a background music area (at 6 o-clock) that shows 3 dots when you hover over it for a second. You can click those dots to turn the background music off or on or click "configure" that will take you to opera://settings/background_music_settings. Also, if you just left-click the area, it'll scroll down the mods page to the mod in question.