This is a handy-dandy animation plug-in similar to Animonger’s AnimSlider, except that it uses frame labels and requires no other setup.
I picked up this style of lip syncing in Flash from Chris Georgenes’ tutorial.
Update (8/26/09): A savvy user caught a stage refresh bug introduced in Flash CS4 with AS3. If you’re using Flash CS4 & AS3 and the symbol isn’t updating when you select a new frame from the panel, download the most recent version below (1.0.3 has the fix).

Update (8/16/10): Version 2.0 now has some great new features, including a “keyframe” mode, audio playback and frame navigation for Flash CS5, and performance improvements. See this post for more details on the updates.
Update (10/5/10): Check out SmartMouth, the new extension that automatically lip syncs.
Update (11/19/10): See Chris Georgenes demo FrameSync in his MAX 2010 Animation session.
Download
(compatible with Flash MX 2004 & up)
FrameSync.zip