How do you get music out of game data?

do you want to rip music out of a disk or iso/rom?

many times this is game-specific. some, like KOTOR 1-2, are in special proprietary archive formats. you have to find an extractor program to remove the actual game files. other times, like with TES 4, they are plain MP3/ogg and you can just copy them out of the install directory.

console games are another matter. from what i know they usually have the music in the same place/format, but it is again a proprietary type/archive. the hard part is getting the files from the iso/rom in order to rip them.
many older games use a type of midi-like file - small size, decent quality.

which game/platform are you looking for?

Such as DDR Extreme (J) Iso. How do I get maximum quality music?

As Zeratul mentioned, this can be very specific per game or game company. A lot of times, Blizzard, puts their music into .mpq files. I always grabbed WoW's music (and StarCraft and Diablo's for that matter) out of an .mpq compiler. It wasn't too hard to find. Some games just put the files onto your computer in a format it can already understand (The Witcher used .ogg for example).