From what I saw in the video you exceeded the size of 3,371,008 bytes, when the music exceeds this limit it presents errors in the tracks during their playback. The solution is to make cuts in the music so that it does not exceed 3,371,008 bytes. Use Audacity to do this, then use the
LoopingAudioConverter to create the bgm file. I always suggest creating a song with a maximum of 2 minutes to keep the music in stereo quality.
See this tutorial -
https://gbatemp.net/threads/custom-theme-thread.377399/page-239#post-6459286
Note: The quality of music does not interfere with the operation of the loop of repetition. The repeat loop only has a problem if the song exceeds 3,371,008 bytes. The power ratio in Hz gives music is to set the sound quality, it does not interfere in the functioning of music, the higher the HZ the better the amount of music. However, the larger the HZ, the larger the file.