This tutorial will explain to you how to use custom music in your game. For a bit of background the first example of music in-game is some prototype with the sound engine. However, here, I'll show how to use an alternative sound engine (that can be used alongside the original, so the base game music still work). The first apparition of this kind of sound is by Irdkwia, in a secret part of his hack ( see the video – spoiler warning – here : https://www.youtube.com/watch?v=hnN0lAobADc ). The second use of this one is by techticks (with the same sound engine) in his City of Light hacks. Please note that this engine isn't perfect, and have the following issue that I'm aware of :

• It only only accept uncompressed sound (but that's probably not a big deal)
• It only support mono sound, while I think the DS support stereo (but that's also probably not of a big deal)
• It have sometimes audio crackling. This is related to some timing issue (I think it's due to the game not providing the data fast enought to the audio process due to how it measure timing). That can be quite problematic. If you want to compare, here is a recording from the game (note that the start time is different of a few second between the two) (played with No$GBA, replacing the intro sequence music) : and here is the original audio ( music from Explorers of Sky, remix by Kamex, original link https://www.youtube.com/watch?v=EAH8-V455p0 )
• It does loop audio, but doesn't seems to support seeking a specific point of time when looping. When looping, it play the audio file again, preventing having a music which loop with an intro sequence (or you need to time this manually with script).

All of those point (in particular the last 2) could possibly be fixed with enhancement of the patch (I may try to take a look at that, but I'm not really good when it comes to audio processing).