I used to create flash cartoons with characters lip synced to sounds. I did it
by placing the sound on its own layer and then drawing each frame's mouth
position accurate to the sound in each frame.

I synced it by scrubbing (moving) through the frames, and I could hear each
discrete sound for each frame. Now I hear nothing, which makes it very hard to
lip sync! How can I hear the pieces of the .wav in each frame?

No, it's not muted. The sound plays when I hit enter, but I just can't hear it
as I'm scrubbing through. I'm on a different computer, and I wonder if it's
some sort of settings issue. It just functions differently than it did where I
was before.

The sound plays when I hit enter, but I can't stop it before it's finished.
The sound does not play unless I start it on or before the first frame.
I can't hear any sound when dragging through the frames on the timeline.

Are you sure you have your SYNC property to that sound set to Stream? You may
have your SYNC set to Event or Start, but if you want to hear the audio while
scrubbing the playhead, you will need to set the SYNC to Stream.