Like several other posters, I wouldn't torture my cats by singing to them. I do hum since I've found that they find that sound calming. When I'm socializing a feral cat, I don't talk at first as I take in food etc., I hum all the time. After about a week or so of humming, I start talking nonsense in a very low voice. I've also danced with a few of my cats, like Donna, but then I only hum as I dance.