Was just troubleshooting a similar issue with 2 PCs trying to use DCS voice chat.
PC1 was a USB headset/mic/track IR setup and PC2 was a Rift S. The Rift S could NOT hear the USB headset from PC1. The USB headset could hear the Rift S from PC2 without issue.
On the Rift S PC2, I switched the Voice Chat output over to the Realtek Soundcard and tried again. I could now hear the USB headset from PC1 over the Desktop Speakers on PC2.
I then closed out of DCS on the Rift PC, and within windows, selected the Rift S as the default sound out device (it was previously set on the Realtek). I then fired up DCS, selected Rift S as the Voice Chat Output source and now had 2 way comms between both PC's.
Hope that helps!