I have an older but good projector. The projector does not have sound so it does not have ARC. Can I stream say Disney+ and the audio will automatically play through the speakers of the streambar? That would seem logical but I'm not sure
Yes, if the audio stays only with streambar then all you need is regular hdmi to send video to the projector. Just so its clear no other components connected to the projector will have audio through the streambar. Just the apps from Roku streamer built into the roku streambar.