Luz can already change his voice. One of what he considers his best trick was when he imitated Maj. Horton and tricked his CO into releasing a bunch of cows.
But really being able to change his voice-now that's just pure Awesome in Luz's opinion.
Luz does have his limits-he won't prank someone who looks so crushed that a joke would only make things worse.
Note I didn't say someone who 'couldn't take a joke'-because those are Luz's favorite targets.