This engine does not appear to support seamless audio looping. The audio system appears to create a gap in the audio when it restarts an audio file and there is no official or community acknowledgement of the issue or a solution. 

The sample files for the engine are fundamentally wrong for seamless audio loops. The engine comes packaged with mp3 files for Rain and Cave ambient loops. Mp3 files add padding on the head of the file during compression, so they will always have a gap during playback. Mp3 files are not ideal for looping audio for this reason. Immediately upon noticing this, I replaced the files with ogg files I prepared with Audacity expecting to solve the issue.

I believe the engine adds a gap to any looping audio during playback regardless of whether there is a gap in the audio file or the size of the file. I prepared my own seamless looping rain sound effect by editing the sample rain mp3 file and exported it as an ogg file, the final file size was 274kb compared to the original 598kb mp3 file. I tested in Unity, Godot and RPG Maker MV to confirm that the rain loop I created was seamless. During playback in the RPG Paper Maker editor and in a standalone build, there was a gap. I tested both as a "Musics" and a "Background Sound" since both are described to be for looping audio in the documentation, and there was a gap for both. I made an even smaller seamless rain loop of 13.5kb assuming it was potentially an issue where it reloaded the audio each time, but the gap was the same regardless of the abysmal file size. I believe something is wrong with how the engine internally tries to handle looping audio.

There does not appear to be any known solution or any discussion around the issue. No one appears to have seamless looping audio in any project posted on the engine's website. The workaround I noticed developers implementing is to add a fade in the audio so the gap is less abrupt or make the audio file long enough that the player moves to another area before the audio has a chance to loop. There are no mentions, discussion boards or reports about this issue either here or the steam reviews or community page, or in the documentation. 

This issue is present in the current v2.0.11 version available on Itch.io, and I've seen the announcement of a 3.0 version but no list of features that acknowledge this issue. I would like to know if this is a known issue and if it is actively being resolved?

Edit: I have a potential workaround using a separate object dedicated to looping the audio. In a repeating Time event with an interval the length of the audio file, it triggers the "Play a Background Sound" command. This seems to reduce the gap, but not completely remove it.