Pencil has some elementary sound support. To add some sound, create a sound layer. Then select the “Import sound…” command in the File menu under Import and choose a Sound, be sure to select why type of sound (be it WAV or MP3). This will create a triangular key in the sound track at the frame where the timeline scrubbing bar is at. The key indicates where the sound will start to play. You can move this key like any other key. A sound layer does not play any sound when it is deactivated. All sounds can be turned on and off for the whole animation by pressing the sound button in the the time-line controls.
Note that the sound will play the sound until it is finished, even if the last image keyframe was played before the first keyframe for the sound.