About the only thing you can do, short of digging through mod_voicemail.c, is to look in the configs here:
conf/lang/en/vm/sounds.xml

Those are the phrase macros that are used by the voicemail module. I think this is the actual phrase that constructs the audio that is played to the caller:
<macro name="voicemail_record_file_check">

You *COULD* try whittling that down to something like just doing a sleep for 500ms - but I can't guarantee that it will behave the way you want. FYI, this is covered in chapter 6 of the new book. ;) The eBook is only $5 right now... http://bit.ly/aijtAC

-MC