When a long sound is played, its volume randomly increases until another sound is heard. Volume change is preceded by a distinct crackle. This applies to both custom and default sounds available in the game. 
 
Example of the bug - video.mp4
Audio extracted from the video - video_audio.mp3
Audio visualization - video_audio_visual.png

Sound used in the game - raw_sound.ogg
Raw sound visualization - raw_sound_visual.png