I have been wondering about this issue myself. I had planned to use a pair of inwalls for the main speakers in a music room, as the existing stereo system uses larger sized speakers, and the second pair of home theater speakers is somewhat cluttered. My only experience with in wall occured 40 years ago, when a friend tried to mount a pair of (AR 3As in wall, between his listening room and the garage. WAF here) All theory aside, it did not sound good at all, soundfield collapsed, and generally was a major disappointment. They were promptly removed, and the wall repaneled.
Home theater has the addional benefit of 5+ channels of sound, which could reduce the soundfield collapse issue.
I just hate to cut holes in the wall, and discover I don't care for the result. Anyone out there with an in wall installation who had used free standing speakers who can comment?