The Roku wireless speakers only work on Roku tv's, simply having a roku streamer won't work. The Roku Sound bar seems like the perfect fit, the Projector has hdmi? Is one labeled ARC? Does have an optical output?
If the Projector has the ARC Hdmi, that's the only connection you use, its sends the video from SB apps to the tv and at the same time allows for the audio from the tv to reach the SB. If you only have regular HDMI, then use that for video from the SB, but you'll need to use the optical cable for the audio from the tv to get to the SB.