Are projector screens transparent to sound?

If it is a solid vinyl substance the answer is no. as far as put the speakers behind the screen. But as for taming room acoustics. I would'nt think the front wall would be a problem for reflection of sound. but hey try it and see. I'd be interested in the resualts you get

Screen material is made of thick vinyl. It won't be very transparent acoustically. I don't think there will be much to gain by placing panels behind it. Usind standard screen brackets I would guess there might be 1" max between the wall and the screen material. you will need to shim the screen out if you want to use thicker panels. A piece of recycled foam carpet pad might be just as effective as acoustic panels in that location. Recycled foam is made with various density foam bits which absorb different frequencies. High frequency will be reflected by the screen itself. I would NOT recommend using a perforated screen. This will degrade picture brightness. Moire effects will also occur with DLP and LCD projectors. Perf screens are also much more expensive.

Ok. So i geuss i just put wall treatments to the side of the screen. Are most people using absorption or diffusion on the front wall?

Unless you have a retractable screen and intend to use the panels for music only then yes it is worth a look at, but as said if your screen is to remain down then no use in treatment the sound wont reach.