@razzy Hey sorry didn’t see this before, sigh 
this is actually a problem with perception on playback. Sound is accurately represented while doing sound scrubbing, but playback will lag the more layers and frames you have. Pencil2D is missing a preview mode, so you’re advised to export quick previews at smaller resolutions (use the output size in the movie export window) so you can preview the full animation, and use the sound scrubbing feature to preview short bursts of animation.
There’s more info here under known issues: [Guide] How to import sound into Pencil2D