The Voice is computer generated and relies on phonetical interpretation of spellings. . We tend to forget how complex English can be regarding different pronunciations and emphasis depending on spelling , and even local usage and convention. For instance my dad always jokingly called Burntisland in Scotland Burntis-land. (I assume 'Burnt Island' is correct. ) And there is a village in Kent called Trottiscliffe, which the locals pronounce 'Trosley'. Some local amenities are actually spelled Trosley. I dont know how satnav says these examples.
I find it remarkable that it is still able to speak in a posh English female voice (if thats your setting)
On the last garmin update I did I noticed some data, that possibly shouldnt have been displayed to users, which indicated the maps are updated by Chinese speakers. It appears addresses are often translated into Pinyin (Chinese ,using Latin letters) based on how the name sounds phonetically - to a chinese speaker.
For instance Calais sometimes appears as something like 'kalay' (I cant remember exactly ) I suspect the software for the voice may be reading the pinyin phonetic version ,which may reflect the pronunciation of a Chinese speaker, or other low wage countries such as India. This is fair enough to some extent, would you be prepared to pay UK or American Wages, or only have updates every few years . And the phonetic may be better. Calais might otherwise be read as Cal -Ace although if IIRC is still does not say Calais quite the same way as an English speaker with a bit too much emphasis on the second syllable .But it could be correct to a French speaker.
With my old standalone Garmin if you set Waypoints you could rename then. I was very tempted to configure it so the 'voice' said something naughty , such as "arriving at ' tickle my bottom' on the left"
It may be possible on the Jazz garmin, but of course I am far too mature and sensible to find out.