Took this video some time ago in a hotel bar. I couldn't (and still can't) figure out what was happening with the harmony on the chorus of Mrs. Robinson. It appears that there is only one man singing- where is the harmony coming from? Is it a harmonic voice being generated by his equipment? I don't think it is pre-recorded, the band is not playing to tracks, so I can't imagine there would be one wild vocal track that they have to sync to.
This is the gag that I've been doing at the end of my show- it's different every time because I'm working it with local guys. But these guys had to be the best so far- with no rehearsal they pulled off the "heckling barbershoppers" routine. And their singing was great too. Have a look.