This is doable with Microsoft's mapping software, and DeLorme's, too.
There are applications that allow you to open and edit the voice files
to fix mispronunciations and to just have fun customizing your
device's comments and commands. It appears there are a couple of
applications available to edit Garmin voice files as well:
http://turboccc.wikispaces.com/TTSVoiceEditor
TTSVoiceEditor
Edit and change Garmin VPM voice file (TTS voice file only)
Main Features
Change phrases like "recalculating" to something more enjoyable
Control voice volume (level)
Change voice rate (speed)
Import/Export from/to text files (used for sharing, cleaning and
copying to newer voice file)
How-To
1- Start TTSVoiceEditor.
2- Use File->Open to open a VPM file (TTS type only)
3- Click twice on the word/expression you wish to change
4- Enter the new word/expression. You are not limited to the
original length. See "Hints" below.
5- Click OK to change or Cancel to abort this change
6- Modify as many words/expressions as you wish by repeating
steps 4&5.
7- Optionally, you can change the voice rate, the voice level and
the screen name.
9- Use File->Save to save the new VPM file. Add ".vpm" to your
filename. You still need to use the original .vpm file name.
10- Copy it on your Garmin in the Garmin/Voice directory.
Also
http://turboccc.wikispaces.com/NonTTSVoiceEditor
NonTTSVoiceEditor
Split and Merge Garmin VPM voice file (non-TTS voice file only)
Main Features
Split a Garmin VPM file into a serie of WAV files
Split a TomTom CHK file into a serie of OGG files (Vorbis audio
files)
Preview and play WAV voice samples and phrases
Merge them back into a VPM file
Model Compatibility
Should be compatible with all Garmin non-TTS voice file, except
DrNightmare & Elfred which are encrypted.
How-To
1- Copy a non-TTS VPM voice file from your GPS to your computer
2- Start NonTTSVoiceEditor wizard and follow the directives
3- Select Extract to split the VPM into WAV files
4- Use an external program like Audacity to record/edit the WAV files
5- Select Play to preview the WAV voice samples and phrases
6- Select Merge to join the WAV files into a VPM file
7- Copy the newly modified non-TTS VPM voice file back to your GPS
The program will merge the audio file you have. Make sure they are in
the same format as the original WAV files. All files should be mono
(rather than stereo). They could be 16-bit PCM audio or 4-bit ADPCM
audio at 22 kHz. I provide no conversion for now, but this may be
coming later.