I don't hear my teammates even when they are in a xbox party, with party audio on.

If your teammates are playing on PC they could join you Xbox Party, where you then include party audio in the capture settings. However, you should be able to hear your teammates through the stream if you are using in game chat and ensure you're not in
an Xbox Party.

When I stream over the screen from my Xbox, I can hear the game audio, just not party audio.

Hi!
Maybe we didn't understand well, through the Xbox clips it never captures the party audio.
Maybe you intend to record a clip with the game audio including your teammates' voice chat (no Xbox party)?

Thank you, but that is not what I asked. My problem is ONLY hearing the Party chat. When joining a party, all OTHER audio is suddenly lost. The second I Leave, it returns.

EDIT:

I've just discovered that the issue is only a problem when in a party with others. When in a party alone, I can hear everything fine but my microphone detects no input.

For me, I tend to use strictly party chat when I am communicating with my friends.

If I want to separate the party chat audio from the in-game audio,, I have to make a party in the Xbox App on Windows 10. You don't have to have your xbox even on at this point. Once the party has started in the Xbox App and my buddies join, I can hear them
fine through my xbox headset.

Now once you actually turn on your xbox, and start streaming. The Audio for the game will come through your speakers, and the audio for the Party should remain through the headset, both separate from each other.

Its not the best solution, but its definitely a clever workaround., however its not really that ideal, because it is a pain to work with, because you cannot control the party while streaming within the xbox. You literally have to end the stream and return
into the xbox app to configure the party settings.

Other than that, the only downside is, that any VOIP chat from any games will still want to push through your speakers and not your headset.