As a test, you could shut down and unplug all of the A/V equipment, and see if that makes a difference with the light (you might want to test with both the CM15A and the TM751).
If it works fine with that stuff shut down and unplugged, then the problem is that the signal isn't making it past all of the noisy equipment / signal suckers with enough strength left to trigger the switch.
You can either move the CM15A to a different circuit / outlet, or put the A/V stuff on a plug-in filter (filters are pretty inexpensive).
I had a problem recently with three switches that refused to work at night. It turns out that my neighbor across the street had a CFL bulb in one of his outdoor fixtures, that was starting to fail. Generated enough noise to kill three of my switches from all the way across the street. Replaced the bulb and all was fine. Just an example of what noise can do to X10 signals.