This AddOn simply plays custom music (like Pokemon music) during pet battles. To add tracks, follow the instructions in music.lua.
For copyright reasons I can't distribute any Pokemon music with this AddOn, but if you know of some suitable music with a permissive licence (or in the public domain), just leave a comment on Curse or WoW Interface and I'll see if I can package it with future releases.
CAUTION: The included Populate_Music scripts will completely overwrite your existing music.lua file! Make sure you back up any manual changes you make.
Both the Windows and Unix scripts require the ogginfo program from the vorbis-tools package.
On Windows, download vorbis-tools-win32-bin.7z from here, unpack it and copy ogginfo.exe, libogg.dll and libvorbis.dll into the Interface\AddOns\PetBattleMusic\ogginfo folder.
On Mac OSX, you can install vorbis-tools from Homebrew (you don't need to put ogginfo in the AddOn folder like on Windows). On other Unix-like systems, you can probably install it from your package manager.
The Unix script also requires mp3info, available from Homebrew or your package manager.
Alpha Versions, API and Source Code
The source code is now available on GitHub.