Actually, the Auto Attendant, Submenu, and Announcements ARE .wav files. The format is "GSM 6.10,8,000 Hz, Mono"
Once you are logged in via the GUI, go to the Auto Attendant, click on the recording, and it will play on your PC. You then have the option to save it to your hard drive, or import another file and send that to the Messaging.
Save a recording, then open it in an audio editor, so that you can be sure of the format. Then you can make your recordings in the same format and be able to upload them.