I was browsing through Twitter while Spotify was playing in a background tab and I needed to go to Spotify tab to pause the song, so I could hear the Twitter video more clearly. I wish that when a video with sound is played on Twitter my Spotify sound volume gets lowered. Much like apps work on Android, when we play a song on Spotify and watch a video on another app, the background sound gets lowered, so we can hear better what is in active use.
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This suggestion was linked on another site, and I think this is a pretty good idea. Additionally, I think it would be neat if there was a setting (maybe in about:config) that could determine what tab's audio gets played. I know a lot of websites have media that automatically plays (mostly ads), so it would be neat to have an option where new tabs with audio get silenced and you aren't interrupted.
Maybe a custom site list would be possible?