String quartet dream

I was at a string quartet concert. I changed my seat a few times to get a better view, ending up in the first row, where there were people's coats spread out but no-one else sitting.

 The string quartet started behind some object like a couch that muffled their sound.  Later, a trombone was doubling the cello part. Then the first violinist and cellist came out into the audience and were dancing while singing their parts to each other instead of playing their instruments. The cellist was a man and the violinist a woman. They were singing in tune but not very well. The melody of the quartet was the same as the minuet from one of Beethoven's piano sonatas...