You could, but the remote tends to eat batteries while using the headphone jack. You'd also only get stereo if that matters. You might be better off with an HDMI audio extractor. Also, I don't know much about DP, but make sure the cable works from HDMI to DP. I think the normal use for such cables is to go from a computer/laptop DP output to HDMI monitor input.
Roku Community Streaming Expert
Help others find this answer and click "Accept as Solution."
If you appreciate my answer, maybe give me a Kudo.
I am not a Roku employee.