First, you need to use the MyTomTom web based application (get it at tomtom.com/getstarted).
Then, when you connect the unit to the computer, the desktop's MyTomTom icon will have an additional menu option called 'Add community voices'. You click here and one category is Add Voices.
It is here you get to browse your computer for voices (recorded, not computer---those can't be installed). You need a dataXX.vif and a dataXX.chk file for each voice. There may also be a dataXX.bmp file where XX is a number from 01 to 99 as I remember.
Anyway, these are the voice files to brwse for and install.
Once installed, you access them on the unit by Preferences-->Voices-->Switch voice.
You can get these files from another tomtom unit you may have or there are some sites on the internet where they are available.
Here's just one:
TomTom - GPS POI
(You won't be able to install commercially obtained voices as they are locked to the unit first installed upon).