The prompt files should be in one of two places.
1) C:\Program Files\TeleFlow\Languages\[Language Directory]
where [Language Directory] is one of English, French or Spanish. (this assumes TeleFlow is installed on the C: drive, of course)
2) Where you TeleFlow Application is located, in a Vox\[Language Directory]\Prompts directory.
If you can't find them in either of those locations, I would suggest doing a file search for 01.Wav. This should help you locate a directory containing a number of .Wav files, with names like 01.Wav, 1ST.wav, 02.Wav, etc.
Once you have the files, the best way to fix them depends a little on other factors. What hardware is your solution running on?
If it is NMS, for example, I would suggest CoolEdit. The NMS installer comes with a copy of CoolEdit 96. It doesn't install by default, so I would suggest copying the prompt files you want to modify to a different computer, and installing the CoolEdit 96 software there, from NMS installer. (rather than running the installer on the working system, which could cause you some problems) If you look in the extraction directories for an NMS install, you should see a CoolEdit directory, which has the installation files you need.
If not NMS, you would need to locate another audio/sound editor for this task. Audacity (
http://audacity.sourceforge.net/) is a open source free audio editor that might do the trick. Although I haven't used it myself, it appears that it can do the job of reducing the volume of audio files.