Saving iMessage audio messages sent/received with iPhone iPhone automatically deletes voice messages sent and received through iMessage within two minutes of sending one, and within two minutes of listening to one received from someone else.
How can I change the amount of time that iPhone and iOS will wait before deleting sent and received audio messages in Messages?
Go to Settings app > Messages and scroll down to AUDIO MESSAGES and VIDEO MESSAGES Under each one, there is an option labeled Expire. Tap on it and then tap Never to prevent them from being deleted automatically.

Never or two minutes are your only options, though.