G'day Rich, welcome to GPS Power.
There's no reason i'm aware of why a KITT *.vpm with a correctly generated JetMouse *.unl won't work if they're properly placed in nuvi 57's Voice folder. Check you've made the UNL file correctly and the file names are the exactly the same, e.g. 10-KITT_aaron.VPM with 10-KITT_aaron.UNL. If you have the default 'Hide extensions for known file types' still checked in Windows>Control Panel>Folder Options>View you may have inadvertently saved the UNL file as *.unl.txt in NotePad.
If the above doesn't help give more info such as whether the voice appears in the nuvi's voice language selection list.