Before I let the students work in groups I always go over what I expect from them... I usually tell them that the only person that need to hear them speak is their partner... I also tell them that I will turn the lights out if it gets too loud...This saves my voice

I tell them if they see the lights go out, they need to stop speaking and look at me

I usually give them two shots ( I let them know this)... If I have to turn the lights out a third time, then we go back to independent work... I understand that you had to have them working in pairs so maybe the independent working thing wouldn't have worked in you case but I wanted to mainly tell you about the use of a signal... it really helps me
Maybe, you could have had a contest of some sort to keep them working quietly... The groups that work quietly get some sort of reward...