Engadget
Too much height makes dialogue and fine details sound less direct.
Setting up a home sound system is an exciting prospect, but can be complicated. Physics, it seems, has its own opinion on where and how high your speakers should be. Every room is different, and though most are about rectangular or square, you still have to contend with different sizes and furniture layouts.
For the front, left and right speakers in a home sound system, "too high" typically means the speakers are mounted or placed well above your ears when you're sitting down. For the most part, a straight and level pathway from the speakers to your ears is the best way to go.
Usually, the front, right and center channels are forward-firing and high-frequency, which is more directional than bass. If these speakers fire over your head, some of the dialogue detail, cymbals and positional effects are lost.
For example, a bookshelf speaker placed at the highest point will produce plenty of sound, but the rich details will sound less clear or muted. Shifting the speakers down to seated ear height will rectify the problem.
Front speakers do most of the heavy lifting with dialogue, music and on-screen action, so they produce the best results at seated ear level.
Surround speakers are a little more flexible because you can place them above seated ear level, as much as 2 or 3 feet. Installing them too high will blur the separation between sounds from behind or beside you and sounds from overhead.