In Animate 1 is not possible assign lip sync mapping to symbol in main scene timeline.
Only lip sync mapping is possible in drawings cells. Then, what we can do if the drawings mouth are contained into symbol, (ex. mouth symbol)?
I´ve experienced that, if you want map the sound in the symbol drawings in main timeline, the first thing to do is drop the symbol in main timeline and select any symbol frame, then, go to edit/expand symbol. This action shows the drawings indented to symbol. If you map these drawings, nothing happens.
The trick: You must rename these drawing element with another word. (ex. change: mouth_1 to mouthlip_1). Now, go to sound layer right click on it and, in the drop down menu go to lip sync / map lip sync and assign mouthlip_1 to mapping, and the mapping are now assigned correctly to the symbol drawings.
This method works very well. Hope that helps.
Yoryo