For testing follow below steps
1. R390 HDMI out ---> LG Monitor HDMI in
2. LG Monitor 3.5mm audio out ---> Polk Aux in (use 3.5mm stereo (male-male) connector)
This should tell you if your R390 is outputing audio as well. However with above setup you will not get desired 5.1 output.
Once your sure your getting audio out of R390 with above steps, then invest on buying OREI HDA-925. That will extract the 5.1 which can be feed into Polk via HDMI or optical. Follow below steps once you have bought OREI HDA-925.
1. R390 HDMI out ---> OREI HDMI in
2. OREI HDMI (ARC) --> LG Monitor HDMI in
3. OREI HDMI (audio out) / Optical ---> Polk HDMI / Optical
If your Polk had passthrough HDMI out (unfortunately you don't) in that case you don't need OREI HDA-925
hope it helps
Cheers,