The 2.2.2 firmware release notes contains the instructions to upload prerecorded auto attendant greetings:
The pdf is located here:
http://relnotes.ciscosbs.com/UC320W_rn_v2-2-2.pdf
Also from an external user can record an updating greeting by dialing into the UC320W and having the AA answer the call. Then dial the Auto Attendant Prompt Recorder extension (default 398) and the user will be prompted to enter the assigned password. After authentication then the caller can record prompts and save and delete them.