On Steam with the Oculus using the 'Launch in Oculus VR mode' option, with the Rift configured for audio mirroring, I only get audio out of the Rift and not my regular speakers.
If I disconnect the Rift it works fine (so not a problem with the speakers) and I've disabled all other audio outputs in the Windows control panel.
[4.2.0 HF2 Steam] Audio mirroring doesn't work
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Related thread: https://forum.egosoft.com/viewtopic.php?t=395767 Audio set to "default windows device" won't play sound of separate headphones. Apparently audio mirroring (which I didn't try) also won't do it.