Custom Music

Importing custom music is one of the easiest modifications to implement, as it simply involves taking any song and making it compatible with the game. New Super Mario Bros. Wii makes use of the BRSTM format, which is also used by several other Nintendo games.

Finding Music
If the music you want to import is from another game, you can likely find a BRSTM already made out of it on the SmashCustomMusic website. However, it's often unreachable, so it takes a bit of luck.

Prerequisites

 * Audacity (or any other sound editor)
 * BrawlBox
 * BRSAR Patcher

Preparing for Conversion
Open Audacity and press File > Import > Audio

As a first step, apply the Normalize effect by pressing Effect > Normalize. This will make the audio as loud as possible (roughly similar to the original game's volume) without causing distortions, fixing any volume discrepancy between the two channels in the process. Leave the Normalize settings as default and press OK. Below is the difference before and after applying the effect:

If you need to make further improvements to the audio, such as crops, tempo/speed changes, do them before going to the next section. If the audio quality is poor, you can find some tips to improve it here.

I'll finish this at some point.

Importing the Song
Open BrawlBox, press New > Audio > BRSTM Audio Stream, import the WAV, set the loop points if needed then press Convert. Once it is done, press File > Save to save the file.

Patching the BRSAR
Read the readme.txt file included, i cannot be arsed to write this right now.