I believe I should've corrected my update and clarified the steps...
- Download your choice of music streaming app to the TV.
- Download the Roku remote app to your mobile.
- Play a song on the TV.
- Open the Roku remote app.
- Look for the Remote icon at the bottom of the screen and tap it.
- While on the Remote page, tap the Mic icon at the top of the screen.
- (And while your music is playing... ) Say "Turn off display" into your mobile.
Things like this can be frustrating (with all the different apps and setting to 'vet' or adjust to have everything work symbiotically). Let us know what happens.