Sure, Minecraft has their sound and music files saved in Ogg Vorbis format, and holds them in "C:\Users\<windows-user-nme>\AppData\Roaming\.minecraft\resources".
Note: The 'AppData' folder is a hidden folder, so make sure you enable the viewing of hidden folders.
In Windows 7, you can do so by: Organize -> Folder and search options -> View -> "Show hidden files, folders, and drives"
This will vary from user to user, computer to computer, but once you find where yours is stored, you can mod it on your own machine. Once you get the modifications to the quality level you want to represent yourself as a composer, you can find someone in the Minecraft modding community who can help you package up your changes in a way that any Minecraft player could install.
Minecraft only officially supports texture packs, but a sound mod wouldn't be too hard to install.
Best of luck!