I spent 10 years or so playing in a ceilidh band, and there were plenty of occasions when we arrived at a venue, and had to set up with the room full of guests and no opportunity to do a sound check. In the end we came up with a solution that worked really well. We'd get everyone plugged in to the PA and check all of the signals, then start off with the squeezeboxes, bringing them up to the right level, I'd then introduce the acoustic guitar, drums and bass, one by one, ensuring that I got the mix right. The bass player would be out at the back of the room and we'd communicate through some simple hand gestures. When the bass player would come in, I'd go out to the back of the room and he would adjust himself as needed in the PA. Finally I'd pick up my electric guitar and the bass player would fade me up through the PA, as I joined in from the back of the room (Wireless). The whole process took about 3-4 mins. There were times that we actually used this method for the first dance, managing it with a floor full of people
It worked very well and numerous customers commented on how professional it seemed. The most pleasing comments were from staff at venues, who would often comment on how well it worked in comparison to other bands setting up.