So, I thought I ran into a bug last night but it turned out to be "as intended". However, it's very easy to think this is a bug and waste time trying to "fix" it, so I thought I'd mention it here.
When using the "Say" action on an actor, the dialog that opens offers you a space to type in the dialogue spoken, as well as link it to an audio file. Any text typed here will show up on the timeline, so it makes for a good reference point.
However, if there is text in this box, Moviestorm will use it for the lip-synch and completely ignore any attached audio.
For example, if I attach an audio file of a lengthy monologue, then type the first line of said monologue into the text box (what I actually did, and why I'm posting this) just for reference, then the actor will mouth that first line of dialogue, and that first line only. They will not synch to the audio at all.
If you leave the text box blank, however, Moviestorm will synch to the audio file instead. Best practice, obviously, is to have both text and audio match exactly... but sometimes this is not feasible, so it's good to know the default behavior in such cases.