Very strange. It does indeed sound like a problem with the audio when another player joins but it's not a global issue as I've just been playing with a few folks and I could hear them fine. Next time it happens, it may be worth checking what your default audio device is set to - sometimes the headset can do some odd things with re-routing the audio playback, etc. To do so, right click on the volume control in the task bar and select 'Playback Devices'. The default playback device should have a green tick next to it.
The DX11 and DX12 versions use a slightly different version of XAudio (as DX12 support guarantees Windows 10 so we can use a later version of the library) so it may be worth switching and seeing whether it still happens in the other version.