From your post, I'm guessing that the buzzing occurs even when not
using the icom ... right? The problem as I see it is that icom
manufacturers seem to refuse to use balanced lines to the stations.
There is usually a 'hot' input to the amplifier run to each station to
the master where the amplifier usually resides. Station speakers are
usually connected to the output of the amplifier, so they can be called.
A lot depends on the particular design of your unit. I recently
purchased a simple, now discontinued, NuTone icom/doorbell. Nice idea.
It provides 2 indoor stations and 2 door stations with an electronic
chime. But, if I put my ear close to the speaker, even when not using
it, I hear a slight buzz. In my case it's not enough to hear, even in a
very quiet house. But, in the instructions, there were all kinds of
warnings to not run the wires close to electrical wires, etc. Yeah
right, that's totally impossible. Even for this simple unit, it was
impossible. When it is in use, you definitely hear some 60Hz buzz in
the speakers. My plan is to someday redesign it but there are other
priorities. Also, as an icom, it's not really that good. Unless you
are close to the speaker, it's hard to understand. I previously
designed an icom for my previous house that worked very well. There was
no hum or buzzing and sound was very clear.
As to your system, do you use it as an intercom or just as a radio, as
many people do? If you don't use the icom features, it might work to
remove the input to the amplifier at the master. Filtering would be
tough because it is harmonics of 60Hz which are in the same band as
voice, that you are hearing. Different ballasts, i.e. electronic
"ballasts" might help also. Electronic ballasts are actually a
switching power supply at a higher frequency. You might try to replace
the ballast on one light that produces the most noise with all the other
lights off. If this works, you might have a chance.