Speaker placement will probably affect the volume more than the loss in the wire. If you are using the M1 for Fire then you want to make sure that any location in the home/business with the doors closed and whatever the normal ambient sound level is will not mask the siren.
In my home (a split with 4 levels) I have 2 speakers installed (one on lower level adjacent to the panel and one adjacent to a junction box in my sons bedroom closet on the top floor where all my upstairs smokes, co's and motions tie into a zone expander)
On the main level its not that loud so I will have to install a third speaker one of these days. Unfortunately I did not plan quite right and I do not have a spare pair of 18 or better wire run to that level from the basement CAN. After replacing almost all of the sheetrock in my house (over 150 sheets) I am not opening the walls again. I have to get creative or ELK will need to design a Wireless Siren (hint hint). Temporarily I plan on having an output from the M1 trigger a siren in my intercom. Not exactly what the NFPA would call Kosher but it will have to do until I can think of something more suitable.
ALWAYS ALWAY ALWAYS pull as much wire as you can fit in your walls........... you will need it some day.