My husband walks around singing familiar tunes like 'It's a Wonderful World' but replaces all the words with the name of one of our cats. It varies as to which name he uses but it is so funny. I can hear him in the shower singing away or while he's vacuuming or something and whoever's name he is using (Stinky for example) will watch him intently, as if they know the song is specially for them. I think it is whatever cat he happens to spot right when he feels like bursting into song.
I razz him about it all the time and he doesn't even realize he's doing it. I always tell him, "your friends would give you such a hard time if they caught you singing to our cats!!" LOL