Minecraft wiki entry for Development_versions, associated text: Five new note block sounds have been added: "iron xylophone", "cow bell", "didgeridoo", "bit", and "banjo".
The new sounds can be heard by using iron blocks, soul sand, pumpkins, emerald blocks, and hay bales respectively.
A previously unused note block sound has now been fully implemented: "pling".
Can be played using glowstone.