Had this problem on an old Garmin 550 and the SRC ... there was a hack to x2 the volume
This is what they did
First, back-up the file you want to experiment with...then you need a hex editor.
Open the VPM voice file with the hex editor, look for a string
< voice_volume>65< /voice_volume>
Note: in reality, the string will not have a space after the "<" - had to do that to make it show up for illustrative purposes
Change that number to something higher, and save the file to your GPSr...you should have a louder voice for the particular file you've changed.
Might help or not !