I'm having this same issue, i have the party chat unmuted on the twitch app but it does not give the people in my party the option to shares their audio.

Please fix this bug or bring back the feature where you can share the audio of your Xbox live party chat that way my stream’s viewers can hear who I’m talking to!!!!

I'm having the same exact issue. My friends and I noticed right away that they could only hear my audio but not themselves. I stream off my xbox and it seems that through the app, I cannot change the "party chat" option to include my friends audio. Usually once I start streaming, there would be an option for the users to include their audio as well. But instead there is no option. This quickly became an issue only because I was the only one who was talking in stream. I've been looking for many solutions through xbox, twitch, microsoft, etc. But anything nearly helpful would be older 5 years or just insensible/unhelpful. Does anyone have a solution?