Inputs
Mikes, piano, keyboard, guitars are all inputs. Anything plugged into the back of the soundboard (at least on our's inputs are in the back). Our soundboard can be moved. We had a loose mike input cable, and when the soundboard was bumped, the wire didn't make a clean connection and would pop in and out.
Unless someone bumped the soundboard, I would rule out the connections based on what you described. How old is the soundboard? Maybe take it to a electonics shop for evaluation. Could be internal. If it really old, dirty/corroded pot or slider. I guessing now.
Easiest to check are the inputs. Unplug one item. Remove sleeve, visually check each wire, slight tug. Each should be clean and solid. If all are ok, possibly check other end (mike end). If that doesn't turn up anything get an "expert".
Good luck.
