Is the spdif the only option or is this a roku stick going into the tv where you can control your volume via the tv and use the headphone out jack from the tv? If so, then yeah a compact mixer from Behringer would get the job done. This one's on sale from Sweetwater:
https://www.sweetwater.com/store/det...mixer-with-usb
Similar to what Mr. Moon has used in the past (I'm sure he's upgraded since then). This mixer doesn't have a SPDIF in so you would either need an adapter for that or if you're just using headphone jack's you will need a 1/8th inch (headphone jack) to 1/4 inch (pro audio standard) adapter to be able to plug into two separate channels, mix them and monitor via the headphone out on the behringer.
The great thing about these mixers (behringer being known as a low end brand generally) is that each channel has its own mic preamp and compressor. Compression is what streamers use so that if they scream into the mic, it caps the amount of output so it doesn't blow out the listener's eardrums. It evens out the peaks in audio output.