@blehbleh Please try with a different sound in WAV format to rule out the possibility of a software or hardware issue.
Pencil2D only accepts for now WAV and MP3 files, but mp3 files sometimes come codified in a way that Pencil2D cannot playback.
If you want you can also upload the file and paste a link to it here so we can take a look to see if it’s a problem that occurs on your computer or if the file has a problem itself.
Also please mention what is your current Operating System.