I don't have a Soundbar. On my Airplay-compatible Roku TV and Streaming Stick+ Airplay is set up via Settings > Apple AirPlay and HomeKit. I expect the Sound bar probably has a similar menu option.
Roku Community Streaming Expert I am not a Roku employee, only a user like you. Please, no support questions via private message -- post them publicly to the Community where others may benefit as well. If this post solves your problem please help others find this answer by clicking "Accept as Solution.".
Streaming Stick 4K+ 3821 | Streaming Stick+ 3810 | TCL Roku TV 43S245/C107X