Thanks Genete for the tip. That works if you are only moving from one shape to another one but becomes more complicated when there is more than 1 in the switch folder as there normally is in lip synching for instance.
If you have ASPro you might find this thread useful... its about keyframing actions by script - i.e. using a switch layer insert mouth shape actions into your time line...
I tend to set up all of my mouth shapes as single frame actions now instead of switch layers (although the only way to do this is to build/rig a mouth that is versatile enough to emote all the different shapes without really switching to a totally different shape)...
Hope that helps, check out my 'share your work' threads for some results of talking tests...