Depending how you have your AA setup
I upload them using "System Audio Files Updates" and then selecting "Mailbox Greeting" then selecting the mailbox number of menu node
Note if it is a new menu node, you will still need to call the voicemail system and do the initial setup (recording pin, name and greeting, which I leave silent at this point)
Make sure your audio files are formatted correctly
I recall some of the older software versions are really painful to upload with (I'm on version MiVB 7.2)