I believe the volume on the voice prompt you downloaded was low. I have tried a number of various voices and found there is a considerable difference in their volumes. They should be better when you finally put them in your unit.
I also have used the MP3 editor but instead of recording my voice, I copied snippets from audio file (music) and installed them. Keep these very short or they can be come annoying.
I have a Nuvi 660 and also find turning on the FM transmitter and putting the sound through the car stereo very good. You can then adjust the volume as loud as you want.