Build a guest return without the guest
Identify the call application’s input. It should get your voice and any sound the guest needs, but not the guest’s own voice. Loopback on its own doesn’t leave their voice out. Use the audio routing builder to name those destinations.
Mute the guest source in the return while leaving it in the stream mix. Ask them to speak again. If the delayed copy disappears, preserve that mix-minus route instead of turning down the audience mix.
Check the room and monitoring paths
Use headphones at both ends for the next test. If the echo stops, sound from a speaker was likely entering a microphone. If it continues, inspect software routes that recapture the call output.
In OBS, check the monitoring device and each source’s monitor setting. The monitor destination must not enter the same return again. Monitoring without echo covers your own headphones.
Have the guest check the return
Speak separately, then together. Play any music or game sound they need and ask whether they hear it once. Record the stream mix as a separate check; a clean sound for the guest doesn’t mean the audience hears a clean mix.