Artison
No other company understands on-wall speakers quite like Artison. They use very high quality components typically reserved for audiophile quality 2-channel speakers in an ingenious package. Using dual adjustable tweeters, each speaker can be wired as a pair, allowing for a front left/right and center in just two speakers.
In addition, their included mounting brackets allow for mounting flush with any flat-panel on the market. They also make grills to fit ANY current model TV. They can add audiophile quality sound to your flat-panel display and make it looks like factory speaker.
Artison also offers on-wall surround sound speaker, in-wall subwoofers, and single speaker surround packages. If you are looking for a stealth installation but don't want to sacrifice sound quality, Artison should be on your list of products.