The nav voice is much louder when it comes on and can be quite jarring, but I'm sure there is a setting somewhere for that...
Use the steering-wheel volume controls when she's (TomTom) speaking to adjust it. I had the opposite problem, the TomTom was too low to hear. You can press the center touchscreen button on the nav screen I think to open a slider for the volume, but it didn't make it very loud even at 100%. Oddly, the steering wheel volume seems to override that touchscreen button (but only when the nav is speaking) and adjusts it further.
You can also press the TALK button on the wheel, say "Navigation," and say "Volume 50%" or something like that I believe as well...
EDIT: Apologies, I see CX_CO already explained
