Last week, I ordered an AMX-2A audio mixer from Vern Little at Vx-aviation.com This little (built into a d-Sub connector) device allows easy input of up to 10 audio inputs (alert tones, music, whatever) into an intercom. He also has some other electronics on his site.
I had a 'brain cramp' and didn't understand the wiring and emailed Vern several times. I got some emails that took a fair amount of time for him to compose and solved my problems, as well as offered some great options. I felt Vern gave me some excellent service and was worthy of a note.
By the way, at $160 postpaid, this is a bargain considering the other options (not many) available to tie in multiple audio inputs. And no, I have no interest or connection to Vern or his company - Maybe I should have put this plug in before I ordered