Stero, proc rates usually do not go hither than 25% for races that have really high rates. Sounds would be things that are REALLY short usually unless they are part of a longer effect. For example the fire sound you hear from the hell hunter fire nade is 100% custom. The sounds you hear for ult blocking and respawning (for races that have been updated with them) are short as they occur often and need to indicate something to the user.
Remember that the point of soundeffects are either to increase immersion or to accent and existing effect.