We already have push-to-talk and audio MMS messages, but leave it to Apple to try and reinvent - and re-patent - their own system. According to a new patent application made in November 2009, Apple envisage a setup whereby audio notes are transmitted via the voice channel rather than a carrier's data backend server, thus removing one potential point of overload. The outgoing message could be recorded audio or it could be text automatically converted into audio for transmission through the voice channel.
"For example, once received [by an iPhone], the vocalized phone number can be passed to an audio output device that (in the case of a speaker) generates an audible rendition of the vocalized phone number. In another case, the vocalized phone number is forwarded to a voice mail server where the receiver records the vocalized phone number as a voice mail message for subsequent playback." Apple patent application
At the other end, the recipient's device could then handle the audio message in various ways, depending on how it has been configured. Most straightforward would be playback via the speaker, as with push-to-talk, but Apple also describe a system whereby the audio messages could be recorded in a visual-voicemail-style app for ad-hoc playback.
Alternatively, they could be transcribed back into text for reading on-screen. Apple reckon the setup could be a boon for avoiding situations where people want to check their messages but looking at the display isn't necessarily safe, such as while driving.