What size speaker? Several options:
1. use a flat panel articulating arm mount.
2. If the ceiling is close, you can use a speaker mount, off of the ceiling.
3. Bridge the corner with blocking. Place blocking flush with top of the screen. Build angled shelf on top of blocking. Place speaker on shelf. You could also hang the screen off of this blocking.
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I just mounted a Da-Lite electric screen over a center channel and my front channels, I custom ordered and added an acoustic perforated drop to clear the speakers, I can partially raise the screen so the screen covers the speakers, it makes a huge difference and the sound is not at all clear even if turned up. They do make a perforated screen but I would think you would then lose picture quality. I would try the speaker on the ceiling as well as below the screen if possible to see the difference. You could always go to a high quality small speaker like a Bose, I have used a Bose 5.1 system in the past all ceiling mounted and they are pretty amazing for being so small and the wife likes that they are so small so not very noticeable., Consumer Reports rates a small Samsung speaker higher than the Bose, but it is all in the ear of listener..