I am a new user to a HP iPAQ 6910/6915 PDA computer with TomTom Navigator 6.
I wanted to add some new voices as the current ones were too boring.
So I spent 2 frustrating hours of it not working. But finally I got it!
First of all, store the voices on the actual TomTom card, NOT the PDA! This will most certainly require a card reader (it's in most laptops as standard equipment)
Second of all, you may NOT! rename your voice files. This will screw them over and they won't work.
Otherwise, I'm pretty sure the main topic has a more detailed installation guide, but these are some of the mistakes I made.
I hope this will help some of you!