Party chat can't include their audio

When you stream through Xbox there is a setting in the twitch app to include party chat. But the party members have to allow their audio to be used. If you have “Party Chat” muted on the broadcast screen (it's in between “Microphone” and “Game”), or won't have the option to include audio.

On Mixer they have to include their audio in the party chat settings.

this feature have been added to Mixer, but not sure about Twitch

Mixer only has a setting to include xbox party chat audio on stream, not game chat audio

In your party settings you have to include party audio and the people in your party have to click of the checkbox in the party settings as well to include their audio. Can't find the direct help link but that sounds like the simplest solution from what
you're describing.

Everyone in the party who is not streaming needs to give permission for their audio to be used for the broadcast. Press the Xbox button to open the guide. At the top of the Party chat options check the box labeled “Include my audio”.