well i use WIM for my /w but just have game sounds on for ready checks, just set you wow sounds to play in the background then even if your tabbed out you will still hear everything in game
what they said, also keep in mind that it will play all sounds when tabbed out so you might want to set your in game music down (if it isn't already) so that you hear the ready check over the music.
I'm pretty sure this can be done with WeakAuras. It has sound settings for which channels to use (Master, music, ambience, etc.).
There's an example of an "Incoming Whisper" aura here: http://www.wowace.com/addons/weakauras/pages/examples/
You'd have to modify it though.