Roku stick connected to display-remote volume control?

Hello! I have a Roku 4K Streaming stick connected to a 40” Seiki Pro SM40UNP display. The display is capable of 4K via DP1.2 but HDMI is only 1.4. Found out setting it to “manual” 4K HDR doesn’t prevent HDCP 2.2 errors. I can deal with this though. 

My issue now is control of volume with the Roku remote; I have a pair of computer speakers hooked up to the display via its headphone out jack. It does supply audio but is not controllable via the Roku remote. Is there a way to get this to work? Thanks in advance!