but I got the wireless speakers for the great sound, especially the sound of music.
Ah, copy that. But as mentioned, Roku devices cannot multi-task. So you can't leave an audio channel running and switch to something else for the video. You'll have to use different devices in some configuration for what you want to do.
Dan Roku Stick (3600), Ultra (4640), Ultra (4670), Premiere (3920), Insignia 720p Roku TV, Sharp 4K Roku TV, Nvidia Shield, Windows 10 Pro x64 running Serviio and Plex on a wired Gigabit network.