Well, theoretically, any USB 7.1 sound card would work. It just needs to be USB powered, have a S/PDIF input and a 7.1 channels 3.5mm jacks output. The main issue would be if it's compatible or not, as if it uses USB for anything else than just power it probably requires additional software, a thing you can't install on a PS4/XBO.
That's also probably one of the main reasons behind the included external audio cards with some headsets, other than just increasing cost because it's a branded USB audio card
It's proven to be compatible and (usually) harassment-free when plugging in everything.