The will not sound good, and I have heard them used as you describe.
Century Stereo (in San Jose, CA) had a pair of Salons in large receases/enclosures in the wall, which had a fabric covered door. They used them in their premium home theater room. While they worked okay for HT; for music, they really did not work. The soundstaging and imaging was completely gone. The frequency response was ruined as well, as the enclosure rolled off both the treble and the bass.
My advise; Don't do it! Either find another way to make them work out into the room, move them to another room, or just sell them and get in the wall or ceiling speakers.