It is possible to use custom navigation voices in the Navigation function. This method shows how to convert voice files from TomTom application to the format usable by Locus Map.
Firstly, you need to have some TomTom voice file. One usable site with a lot of free voices is for example here >>.
Copy *.VIF and *.ZIP (you can also *.BMP) files directly into Locus/data/nav_audio directory in your phone.
Complete list of voice .ogg files, that should be in the packed .zip file: