There's dictation, there's Siri, and there's audio messages. There's really nothing called voice texting, and that's where at least some of the confusion comes from.
Dictation is basically the microphone button that appears on the keyboard that can be used to type something in by dictating. Right now it's attached to Siri in the sense that's it's there when Siri is enabled and it's not there when Siri is disabled. In iOS 8.1 it should be separate in the sense that you can disable it while still having Siri enabled, for example.
Siri is, well, Siri. You can use it to text people, to read messages, to email people and read email messages, check your calendar, the weather, sports, etc., etc., etc. In iOS 8.1 you can still do that while you can separately disable dictation form the keyboard if you don't really use that or don't want that key there basically.
Audio messages are separate from all that and basically provide a way to record a message and send it as an audio recording so that the recipient can listen to it.