I noticed that no overworld music plays after you beat a level that ends with a boss battle (or any battle that ends when all sprites are gone). I want to get rid of this - how can I do it?.
Answer: Change the byte at 0x2102E to "80" to fix this.
From the FAQ. Search 0x2102E and change what's there to 80.
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As for Mario's Nightmare Quest? Well, it's currently on Fusion Gameworks, ROM Hacking.net or the GCN at the moment.