I almost feel silly posting this, but there has to be an easy answer.
My parents had a programmable doorbell. It died, they replaced with the iChime (see manual: http://www.ichime.com/iChimeManual.pdf
for reference). It had two extra speakers (maybe three...there's one outside at the front door, one in part of the house, and one outside in their back year) already wired in. Those speakers work, but the volume on the iChime is quite a bit lower than the doorbell they had. They want to add an extra speaker to the system. (House is L shaped. Main doorbell is at the 90 degree point. One speaker is at one end, they want a speaker at the other.)
So, I assume you would need an amp? What kind of amp do you use to power three (four?) mono doorbell speakers? Where does one even source extra doorbell speakers and amps for them?
Or is there some better way to solve this problem? They want something they can program and can hear. They would be willing to re-replace the iChime if something else worked better.