Use .wav files for the best stability and to avoid syncing issues.
You must include the BPM in the file name (e.g., MySong_90bpm.wav ) so the game can detect the rhythm. BPM.BULLETS.PER.MINUTE.Custom.Music-DOGE.part1.rar
Use free software like Audacity to trim your tracks or adjust their tempo to fit the game's metronome. BPM.BULLETS.PER.MINUTE.Custom.Music-DOGE.part1.rar