YouTube don't release sound card
When I closed tab with launched YouTube video, sound card stays occupied with Firefox, so I can't use exclusive mode with my sound card, such as ASIO for ex. Closing Firefox helps only. Other websites with audio content, including other html5-based videoplayers, don't make this problem.
Alla svar (1)
Try https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-using-safe-mode As well if possible update your sound card drivers from the Manuf. Be aware Windows 10 can and will change your hardware drivers to their own drivers. https://www.howtogeek.com/302595/how-to-stop-windows-10-from-automatically-updating-hardware-drivers/
I am looking but everything is archived and will not work for your version. Frustration at it's best.
Please let us know if this solved your issue or if need further assistance.