FocusInterruptSounds
Makes a sound when your current target (or focus**) starts casting. To configure settings type '/fis' (or go to Interface Options).
Also has basic functionality to announce successful interrupts.
** Use with [target=focus] macros for interrupts (Counterspell, Spell Lock, Wind Shear, ...).
I'm running version r 40 with WoW 3.0.8 and I'm not having any problems.
For those of you who may not be familiar with this addon, it makes a sound when your target is casting something that it thinks you might want to interrupt, but what I like the most is that it also makes a (different) sound when you successfully interrupt your target's spell in time.
Happy interrupting :D