how do become a theatre sound system or home theatre installer?

Look into CEDIA - an organization that holds classes and certifies home theater installers.

You might also look into ISF to become a calibrationist.

Lucas arts also has training to become a THX certified installer.

Installing gear is very different from being a Tech. Woodwoorking, cable pulling, sheet-rock patching are actually more important. If you want some certifications - Look into the following: ISF Calibrationists - These guys calibrate HDTV's and projectors to industry standards. CEDIA - Certified Electronic Design Installers Association - An organization for home-theater installers to join, take classes and learn how to do design, acoustics, finishing, etc. THX - they have a certification course to teach you how to install and make the room follow THX standards. NOTE: Right now the economy is bad. Un-employment is over 10%. The high-end jobs ($100,000) are always rare and the middle-class are not installing dedicated HT systems. What I am saying - dont bet your future on this 'luxury' industry right now. Take some classes, learn, but dont count on this to make a living right now. (Just my opinion).

I'm actually doing alot of research in that as well. Are you from Canada or US? Also try http://espa.org/

They have different levels of certifications. These aren't only for home theaters, you will be certified to install security systems and other types of home electronics. Actually, if you stumble upon any information, could you please let me know? I'm trying to do the same thing haha