When I try to send audio message on my iPad Air I press mic button and when I try to send or playback for myself it just disappears. It has always worked in the past.
1. Initiate the iMessage app
2. Insert the recipient
3. Press and hold the mic icon to the right of the text input area and speak your message.
4. Afterwards, swipe up to the arrow to send it.
First, try a reboot of the iPad... If that doesn't help... Consider hooking up to iTunes, sync/back-up... Restore the iOS... Then restore the back-up and sync to iTunes.