If you break a block and don't pick up the item and break another block with a different tool with the same ID, it sometimes doesn't play the noise of the block breaking if you don't pick up the item it drops.
The easiest way to reproduce this is:
1. Place a line of two alternating types of rails.
2. Get a tool (I used an iron axe) and break each one without picking up the dropped items.
Another way:
1. Get two diamond pickaxes with different enchantments on them (one must have silk touch, the other must not.
2. Strip mine, alternating each pickaxe with each block you break. Don't pick up the items.
Is this in single player or multiplayer? If multiplayer, is it vanilla or a Bukkit server?