I was thinking about this today, so it's really cool that someone posted this!!
I tend to sing A LOT if I'm having some mental issues because it grounds me. During lap time, I'll start to sing and Santino will come rushing over to me and Barney makes a couple wheeks (he's my backup

). My cat Dora seems to think that my singing is me calling her, so she rushes over as well, although if a pig is in my lap, she's not allowed there, but she'll sit at my feet and rub, rub, rub.
It's strange how they react to singing and music in general. If I sing something soothing like Etta James or The Counting Crows (circa '94) then everyone just kind of chills and lay about. Once the B52s start, there is a lot of movement, popcorning, etc... No one really seems to respond when I play Marilyn Manson...they just mill about doing their own thing...
I've started dancing and singing with the pigs like I do with Dora and I get a reward made of purrs
I LOVE IT!!!