The supported audio format for playing audio files on the Avaya Aura Media Server is linear 16-bit PCM 8KHZ mono in a wav container. A good open-source tool for recording such announcements is Audacity. By default it will record in stereo, so make sure you set it to mono recording.
We've got it on our roadmap to allow recording of announcements via placing a call into the system (or having the system call you). However, this is not yet a committed feature.