She started to scream, at first, crying and fussing and fidgeting, though the rocking helped and... once he started to sing, softly, she started to settle down a bit, crying just softly, as if she really wanted to cry a lot but his singing was so soft she couldn't cry too loud or she might miss it.

After a moment she'd let out a single loud cry, then be soft again, angry face scrunched up, fists balled up, but not loud. As long as he kept singing.