Minecraft wiki entry for Development_versions, associated text: New mood detection algorithm for cave sounds.
Aims to make ambient cave sounds play at more appropriate moments.
Mood increases the longer a player remains underground.
Mood increases the longer a player remains without or in low sky light or block light.[more information needed]
Mood decreases and eventually resets the longer a player is neither underground nor in the absence of light.
You can check the percentage of the mood in the sounds info section in the debug screen.
When the mood percentage reaches 100%, an ambient cave sound will play.