Hello, Todd.
Thank you for visiting Apple Support Communities.
I understand that you need access to some audio files that were sent through iMessage. Audio messages sent through iMessage are set to automatically expire after two minutes unless you actively choose to save them. If you did not save them by tapping “Keep,” you will not be able to retrieve them at this point, but in the future, you can save important audio messages by tapping Keep or adjust your settings to always save the audio and video messages:
An audio or video message recorded and sent from Messages will expire two minutes after you play it. Before an audio or video message expires, you can tap Keep, under the message, to manually save it in Messages and to your attachments. To view your saved attachments, tap Details while viewing the conversation.
You can adjust your Messages settings so that your device will automatically save all audio and video messages. Go to Settings > Messages and change the setting for audio or video messages.
You can't keep audio and video messages longer than you keep your message history. Tap Settings > Messages > Keep Messages and increase this setting if you want to keep audio and video messages longer.
Send audio, photo, and video messages on your iPhone, iPad, or iPod touch with iOS 8
Cheers,
Jason H.