Not sure if it'd work, but many aftermarket head units have what is
called an "audio interrupt" primarily designed for in-car cell phones
that kills the audio. I'm not sure how it works on the cell phone end
but its just one wire in the back of the radio. You might look into this
route and see if it will work on the CB.

Brandonb

Olds used to have something like that.  They offered an in dash AM/FM/CB
back in the '80s.  When a CB signal higher than your squelch level came
in, it overrode the radio.

Dunno how to do it now though.  Suggest going to Radio Shack and asking.

FrankW did pass the time by typing:

Yep.

For a good speaker seek out the RadioShack Minimus 7.  It's a
waterproof exterior speaker in an aluminum box.  Works well
on my friends CB.

As for cutting into a speaker, that's tricky.  It requires you
keep the load balanced for the radio (some may do fine with
missing loads but it causes grief on some amplifier circuits)

Too bad you weren't asking about a system for your bike.
This one is slick. http://www.autocom.co.uk/

A diy version. http://www.ldrider.ca/techpages/audiooverridebox.htm

Might be a starting point.

Me, I'd go with a separate speaker and possibly a mute
for the stereo (some aftermarket and OEM have a "cellular" line
that you ground to make the radio mute)