Are projector screens transparent to sound?

Agree with CMK. Get a Stewart 1.3 gain with the wide Delux frame and Velux coating. The extra wide frame looks much more finished than square tube frames. Overspray hits the frame and is absorbed by the special Velux material. After your eyes adjust to the dark you can see the overspray. Velux helps this problem. It will be the last screen you buy unless you change size.

Thanks, Guys.
Any suggestions on where to get a good price on a stewart?

AVScience has competive pricing on Stewart screens.

While Stewart doesn't make an inexpensive screen, the price difference may not be too interesting once you've specified the useful options : wide frame, light absorbing coating, and the 1.3 gain fabric.

I paid just a couple hundred dollars more for my 87x49" Velux Deluxe Stewart with Studiotek 130 than I would have a DaLite with the same options.

Also note that the big manufacturers including Stewart will build a screen to any size and shape you want. If you'd prefer 2.4:1 or 1.85:1 they'll do that.

The screen of choice should be the Stewart Screen.
I mounted mine away from the wall and have sound panels behind the sceen which are not visible but make a profound difference to the room. I also elected to to put sound panels beside the screen in my set up.
Back to the sceen, get the wider frame for the screen the one that has the flocking/velour finish on it. Any reflections will not show up with this frame whereas the metal frame with only the black paint will show the reflections. This would drive me crazy, well maybe crazier.
Good luck.