While the button for sending audio messages in the iOS 8 Messages app is readily apparent, one feature that may not be so obvious is that you can actually record and send audio messages even more quickly by simply raising your iPhone to your ear. For this to work, you need to ensure that the “Raise to Listen” option is enabled under Settings, Messages and your iPhone screen must be on and showing an active iMessage conversation.
As the name of the setting suggests, this feature will also allow you to listen to incoming audio messages in the same manner, allowing you to effectively keep up a back-and-forth conversation without needing to worry about tapping on buttons. Note that Audio Messages are only available in iMessage conversations.
In the same way that the audio button won’t appear in a standard SMS conversation, the raise-to-record feature won’t do anything either if you don’t have an actual iMessage conversation open on your screen. Your audio message should be sent out automatically once you take your iPhone away from your ear, unless it’s a really short message, in which case you’ll need to tap the “Send” button to confirm it, or the “X” to erase it — this is presumably to prevent sending of merely accidental recordings.