At the time of writing this, this process only works on Dolphin Emulator. As such you need an ISO of No More Heroes 2 and Dolphin. Obtain an ISO of NMH2 legally from your own copy.
Now that your have both of the above things, you will need:
NMH2 audio file identification as a reference (https://gamebanana.com/tuts/12512)
Your custom audio as a BRSTM file (if you want to know how to make your file a BRSTM check out a guide on how to do so)
Let us begin!
After downloading and extracting WiiScrubber, the folder should look like (Pic 2).
Run "MakeKeyBin.exe" and follow it's instructions. When you are finished, your folder will look like (Pic 3).
3.Preparing your sound file
Now that your have your BRSTM sound file, rename it to whatever you are replacing it with. Use the sound index to determine which sound is which. For example, if you want to change the "Deathmatch select music", the index tells you this file is named "DEATHMATCH_SELECT.BRSTM". Thus, rename your new custom track to "DEATHMATCH_SELECT.BRSTM".
4.Doing the thing
With all that prep out of the way, now you can add custom music! Open WiiScrubber.exe. Load your NMH2 ISO. From there a tree branch menu will appear. Navigate to Partition 1 (where 99% of the games files are), Sound, Stream. The audio files to replace are in here. To recap:
Find the file you wish to replace. Right click on it. A menu will appear with two options, "Extract" and "Replace". Click "Replace".
Select your custom audio file that has the same name. It will replace the file then refresh. Click "Scrub".
You will get your ISO "[Filename here](Compressed)". Open this file with Dolphin and verify it works. You can use this as your main ISO now. If there are any issues comment below and I will attempt to fix them.