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