Thanks a lot for this excellent post. A lot of ideas for me to use in there, starting with :

/console Sound_EnableSFX 0
/castrandom [combat] Hammer of Wrath, Hammer of the Righteous, Shield of Righteousness
/console Sound_EnableSFX 1
/run UIErrorsFrame:Clear()

OMG, no more annoying "Spell not ready" sound !