Unfortunately we have no recent Nvidia cards here to test with so we can’t replicate your setup.
4K @ 60Hz, even if it would be in 4:2:0, is very unlikely for HDMI 1.4 though! None of us have seen such a configuration work.
Is there no alternative way to route the sound towards your receiver?
We can’t think of any other option to try in your situation. It’s possible that an HDMI 2.0 to DP (1.3 maybe) converter will be introduced at some point but nothing has been confirmed so far.