Yesterday notch twittered about the possibility that we soon will be able to trigger sounds, or tones with redstone. He gave us this highly illustrative picture.
With C, C# and D being tones. And I can simply guess that the box at the bottom is showing that different materials will give off different sound. And ofc course SP + SMP means that it's gonna work on both singel- and multiplayer. Woho! cool stuff!