I love boozle's videos, i've watched them all =D
Sheldon does respond to most of the songs he hears, but he sings along in his own tune. I have been trying to get him to sing "you are my sunshine" and he makes some noise but not in tune yet.
He has also learnt to mimic almost all the wild bird sounds in my area and also to wolf whistle, kiss and make this long depressing droning sound.. no idea where he picked that up XD