My rules of engagement with speakerphones
(1) Tell the other party you're switching to speakerphone '
(2) Tell them who else is listening in on the call at your end;
(3) Check that they're okay with that before proceeding ;
(4) Assume that the person at the other end has gone to speakerphone anyway and hasn't afforded you the same courtesies, so be careful what you say ;
Put another way - always assume your audience is bigger than you think it is, and don't make any assumptions about who's listening in at the other end. Been there, done that, got the T-shirt that says "Sorry I can't sit down, my arse just got kicked big style".
Mine's the one with the armour in the seat region.