there are different ways to make generic vocal sounds, and I suspect they would all yield slightly different velocity results, because some would be more percussive.

For example, singing BA_BA_BAAAA would introduce an increased velocity at the start of the note... which probably would not be suitable for a cello, but might work fine for a mallet instrument or piano.

Different ways to hum include

MMMMmmmmmm
NNNnnnnnnn
Ahhhhhhh
Ooooooooo

and you could introduce a consonant when you want more velocity

Thoughts to consider for anybody who spends time playing with this

https://www.dropbox.com/sh/ey71h8ep047758h/AAC12jNM0Hz9Q8zsh5iDeqNHa?dl=0