@Pencil3d Importing sound from videos (mp4) requires using a different command than importing sound from audio files. Please make sure you’re using the correct option.
Also please take a look at this guide to learn how to import sound and what some of the currently known issues with sound import are: [Guide] How to import sound into Pencil2D
Additionally Pencil2D has released version 0.6.6 recently so we highly recommend you get that version instead of whatever version you’re using right now.