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, ...).

Comments: https://www.curseforge.com/wow/addons/focus-interrupt-sounds
GitHub: https://github.com/corgatag/focusinterruptsounds