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They are currently investigating this! ^_^
https://twitch.uservoice.com/forums/923041-twitch-studio/suggestions/38368711-build-in-audio-mixer-to-be-able-to-mute-programs-o

There's actually another thread for this already! There's a bunch of us that agrees with you, and that thread could definitely use another vote :D

https://twitch.uservoice.com/forums/923041-twitch-studio/suggestions/38368711-build-in-audio-mixer-to-be-able-to-mute-programs-o