Apple added the ability to send audio/voice text messages in iOS 8.0. This is great feature for many reasons, but it is also very easy to send an audio recording by accident. Unfortunately there is no way to disable this feature without a jailbroken device. iPhone owners who are jailbroken can use the tweak NoAudioRecorder to disable this feature. The tweak is available on Cydia for free.
One good thing about the feature is they're set automatically set to expire after a certain period of time. You can disable this self destruct feature by going to Settings > Messages and tapping the the Expire panel under Audio Messages. From here you can set your audio messages to expire in "2 Minutes" or "Never."