If you want to replace the decoder, you must also write your own full replacement for the player software, too.
You could replace the decoder without replacing the whole player software. With some clever code in the kernel you could intercept calls to the file system, allowing you to lie to the player when it reads the metadata (telling it that all the MP3 files are WAV files). Then you could intercept calls to load the MP3s from disk and decode them to WAV on the fly.
Not easy, but maybe easier than rewriting the whole player.
Anyway, I can't see any good reason to replace the decoder or the player software, both are excellent.
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Remind me to change my signature to something more interesting someday