Well, it doesn't exist yet. I have to write it first. If you don't mind editing the script yourself, you can get it to skip the extensions by inserting the following two lines in the voiceUtils.vbs file.:
LastDot = InStrRev(ClipSpeak,".",-1,1)
ClipSpeak = Left(ClipSpeak,LastDot-1)
These should go right after the line that says
Log_file.WriteLine("File: "&Item&"\"&ClipSpeak&" voiced")
and before the line that says
Call TextToMP3(ClipSpeak,WavFile,TalkFile)
This doesn't give you the option to turn extensions on and off yet; that'll require an update to the INI file and its reader, and another if/then. It'll give you what you want right now, OTOH.