As has been reported a few times over the years, Warframe only sends Stereo audio through SPDIF (Optical Out/TOS Link) instead of proper 5.1 surround sound, and I think I know exactly why.
I am specifically using a Sound Blaster X4 external soundcard to encode a Dolby Digital Live audio stream via optical cable to my surround system. This works for literally every other game I have except for Warframe, which only outputs L and R Stereo sound.
I believe this is because Warframe prioritizes sound device jack information (the amount of actual physical channels) to determine the audio out and, due to Windows weirdness, SPDIF is always listed as only having 2 physical channels in the device jack information even if it has 5.1 capabilities. This is what windows sees for SPDIF
In order to get around this, I have to setup a Voicemeter virtual audio cable with 5.1 output that is then routed to my SPDIF out device (the Sound Blaster X4). This is not ideal as it introduces fairly significant audio latency (and its not great to have to download a whole audio system just for one game), but the reason this works is because the Voicemeter virtual audio cable has no associated jack information, so Warframe is likely forced to use whatever the cable is configured for software-wise, which is 5.1. So Warframe is set to output to this virtual audio cable which it sees as 5.1 (so we get 5.1 sound), then using Voicemeter, the audio from the virtual cable is sent to the SPDIF out, which results in proper 5.1 audio from my SPDIF device. I have verified it is proper 5.1, and not just stereo being duplicated to the other channels.
A fix would either be to allow players to force surround sound in the audio settings as an additional setting (with choices like Mono, Stereo, 5.1, 7.1, etc), or to change Warframe to do what every other game I have does which is to use the software output of the device (in this case Dolby Digital Live) instead of basing the audio output on the amount of physical channels windows sees for the device.