He has been doing a lot of phonetic imitating. He doesn't mimic sounds yet, but if I say, "yay yay yay" he'll do the same. And he loves fake laughing. It's really humorous, especially when it is accompanied with his little knee-slap.
I've gotten into the habit (mainly because Owen now looks for it) of saying Yay!! When he claps. He looks at me every time he starts clapping, in expectation of my encouragement and he gives a huge satisfied grin when I say it. Earlier today, when he did something I thought noteworthy, I said Yay! and he lifted one hand, passed the toy he had in one hand to the other, he realized that didn't work so he passed it back only then to put it down - all so he could clap! Haha! I didn't realize the association between clapping and Yay! was so strong until then. He heard the word, and all he could do was make it work to clap. man, he's so cute.