Here is my general workflow for doing a bit of phonemes for animation:
Import handful of PNG files that are drawings of mouths. Pre-compose the selection in a "Mouth" pre-comp. Reduce the images to one frame each. Sequence them in order. So closed mouth is frame 0, slightly open mouth is frame 1, mouth making an "MMM" sound is frame 2... and so on.
Then I go into my main composition, right click the Mouth pre-comp and do Enable Time Remapping. I go to the first keyframe and right click it -> Toggle Hold Keyframe.
This now has my Mouth pre-comp frozen on the closed mouth frame. Then when I need different mouths I can flip through the keyframes as I need. A sentence might have 30 - 60 keyframe changes throughout. This works for what I need and the mouth changes, allowing me to do my speech animation.
Now the bug -------
Seemingly at random the pre-comp will stop honoring my keyframe changes. I will go to a keyframe 2 - mouth open - but it will just stay on the previous keyframe. I have to go into the pre-comp, delete the layer of the mouth that is having an issue, and put it back as it was. A completely arbitrary set of actions but it fixes the issue... at least for a brief amount of time. I'm lucky if I can then render the scene without further issues. And a lot of times when I boot the program back up - the issue is back.