First, which file are you using? In order to find out, go to NBX Messaging, click on Auto Attendant, click on Menu Tree.. you will then see the name of the prompt, for example, aamenu.wav
then you can ftp into the nbx and go to /htfs0/R6_5_22/NBoss/aa/adpcm (presuming you are running 6.5.22 here, if not, just change the version you are running)
If you are running SIP and using g7111, it would be /htfs0/R6_5_22/NBoss/aa/mulaw.
Then you can edit it on your PC.. A word of caution.. I would advise you not change the aamenu.wav file.
Rather, edit it on your pc, rename it to something else, then import it using the NBX Messaging/Auto Attendant/click on one/Prompt.. Then rename it in the AA that you want.. It is much safer that way!!!