From a quick test, it looks like the file is:
/var/lib/asterisk/sounds/a
gent-pass.
<pcm_mode>
Instructions for recording your own prompts are given here:
http://help.trixbox.com/AutoAnswer/How_do_I_Change_the_Default_Voice_Prompt_RecordingsI generally use one of the free audio recording utilities to re-record prompts, then copy them to Asterisk using WinSCP. This sitehas some very good audio utilities:
http://www.nch.com.au