Exactly the sound when taking damage, putting blocks, hitting a mob, receive damage and explosions. Sounds like walking lower it a bit, sounds like picking up items or experience do not affect it.
How to reproduce

• Open a world
• Turn up the ambience volume or music to the maximum (with music the change is more noticeable)
• Don't move for a moment (This will bring the sounds back to their original volume)
• Hit a mob or break a block
• The ambient or music volume decreases
  Note:
  When I mention "decrease" I don't mean the sounds menu, I mean the in-game volume and it is most notable in the last snapshot for the new ambient sounds of nether biomes, where the experience is more affected