@Puga Hi. Please make sure you are using the latest version of the software v0.6.6, any version below this one will not have several of the most recent fixes made to improve the sound playback and former issues. I personally can play the entire song thrice without problem.
I mention the update because you also described having to convert the file to WAV. In the latest version this is done under the hood by Pencil2D, so regardless of audio file you will always get to use WAV internally, however in previous versions the sound file was used “as-is” and some MP3’s have playback issues due to incompatible codec patent licensing with the multimedia plugin (FFMPEG).
Another reason this could be happening is due to your computer hardware specs. It is necessary to have a decent CPU and RAM size in order to have near uninterrupted playback, and that is true for any software.
Additionally, playback in the Pencil2D editor is not always of the final speed of your animation in relation with the audio. Depending on your hardware the software can lag a bit more or a bit less, but if you have plenty of layers and frames it will always lag and your audio perception will be thrown off. In this case we recommend to always export a preview movie or use the audio scrubbing feature to accurately know where the audio is inrelation to a particular frame position.
Now, looking at your file I see all your frames are empty, this is a red flag that you’re using an older version and you should update ASAP.
If by any means you still have the file open and you still see the drawings, DO NOT CLOSE THE PROGRAM, and I beg you please save an additional copy of the file as well as export a sequence of still images with PNG format and transparency enabled to have a hard copy of the drawings. Afterwards please follow our recovery guide to have a chance to get your drawings back in case they are missing from the new file you saved apart.
Note: also consider making a habit of saving separate files by number according to your milestones, never ever save over the same file for a long period of time, this is advice for any software not just Pencil2D