This is one of the many reasons I stay away from Wireless Security Systems. The thing is, the industry is well aware of the problems wireless security systems cause. Unfortuntately the ease of installation and quick monitoring revenues tends to win out over providing a decent system.
I'm not going to profess to being an expert in home theatre, I don't know where the interference is entering the system. If you have RF audio or video transmission I could see that as the culprit. As for changing the frequency, I don't know if this can be done but if so, that might just fix your problem.
Another solution would be to block RF out of the media room, although costly, metal walls might do the trick.
Honestly, I would suggest you contact the company who sold you the system. If you are positive the alarm system is causing the problem, they can test it out by powering the ENTIRE system down. This includes the wireless door contacts, motion sensor and anything else that would be sending that pulse. If the problem goes away, tell them to make the system go away. Opt for a hardwired security system. Yes there is more work involved in the installation, but the quality is better.
http://www.keepsafesystems.ca - KeepSafe Systems installs services and provides alarm monitoring services for security alarm systems in the Vancouver area of the Lower Mainland Fraser Valley British Columbia.
The KeepSafe Systems service area includes but is not limited to North Vancouver, West Vancouver, Richmond, Delta, Tsawassen, Ladner, Surrey, Cloverdale, Abbotsford, Mission, Aldergrove, Maple Ridge, Haney, Port Coquitlam, Coquitlam, Port Moody, Burnaby, New Westminster, Kamloops, Kelowna, Courtney, Comox, Nanaimo, Lions Bay, Squamish and pretty much everywhere in between.