v2016.12.02 Changelog: "Changed mp3 BGMs to oggs! Now the music should play correctly."
1) I think you accidentally overlooked the MP3 file in \Audio\SE\.
- Non-OGG files in \Audio\SE\: question.mp3, cancel4.wav, cursor8.wav, decide2.wav
Basically, what I did was convert all non-OGG audio files (in \Audio\BGM\ & \Audio\SE\) to OGG (constant 128 kbps), so as to achieve smaller filesize & maximum system compatibility.
2) Gameplay Inventory Question
May I check what is the purpose of the 'Key Items' section under ESC > Items menu ? This 'Key Items' section always remains empty no matter how times I play through the entire game (all endings): Screenshots