Elk 120 / 124 Voice Modules & the M1

From reading the installation instructions it appears that these units can't be used in conjunction with the existing audio output from an M1.
 
Am I missing something?

You're not missing anything.  The 124 and 128 are designed to drive their own speakers.  If you want to have a single set of speakers for both the M1 audio and the 124/128 audio, you could run the two outputs through a small mixer amp.

it would have to be a speaker-level mixer tho, right?
unless you know how to get line level out and then to one's own amp! if you do let us know

Here is how I do it.
 
 

ano's suggestion is a nice, inexpensive, off-the-shelf solution.
 
A speaker-level to line-level converter is nothing more than a voltage divider network consisting of a couple of resistors and perhaps a trim pot to allow for adjusting the level.
 
You can see how it's done in this ridiculously overpriced Russound converter.
 
https://www.crutchfield.com/S-M3X6ayaqlmV/p_543ADP12/Russound-ADP-1-2-Speaker-level-to-Line-level-Adapter.html