Here's an example
http://www.av-outlet.com/en-us/dept_257.html
These prices are pretty high but you get the idea. Google around.
Typically for speakers, if the line is already amplified and you just want to connect the speaker to it, "binding posts" are most common and give good connection.
Keep in mind also that to be safe and maintain fire code, you need to use "in-wall" rated wire, and get a decent gauge. With the ceiling being "open" it may be fun routing the wires so oyu don't see them.
There's no specific code for low/non-voltage box height in NC, but for asthetics I'd mount them the same height as your electric boxes.
As for height of speakers themselves, that depends where you want the sound stage and and option of mobility. For movies you want them behind you around ear level. But if you're cranking the stereo and playing pool on the other side of the room and moving around alot, higher (maybe ceiling) would be better.
