Wiktionary
sing up
vb. 1 to sing louder. 2 (context music English) to sing higher, to sing an ascending scale
Usage examples of "sing up".
Thats because you avent seen wot shit-for-brains ere can sing up, Mudge told him as he backed as far away from his tall friend as the fire would permit.
She couldn't quite remember doing it, not with the red haze coating her vision, but she felt the impact sing up her free arm as her fist connected with his face.
He would have to be very, very careful indeed if he tried to sing up anything else, he decided.