Last week a similar thing happened to me. The volume level really faded in the middle of a song. I turned up he master vol on the mixer and carried on. It came back after a minute or two, and I hastily had to turn the vol back.

The thing happened again later, and this time I jiggled with the cables into the mixer. And guess what, I had obviously, in my hurry to set up, not properly seated one of the speaker cables. Phew!