A CODEC is what encodes and/or compresses the digital audio signal into a computer file, as well as allows playback. They don't "run in the background".
If you want a music file type that improves sound quality, try APE, Ogg Vorbis, FLAC or Raw Wave. The last one makes for some humongous file sizes.
Another thing to try is ripping MP3s with a higher bitrate. 128K is supposedly CD-quality. Try 192KB or 256KB. Files will be larger (but not even close to a raw wave) and have better quality sound.
If you're buying/stealing music online, you're at the mercy of the provider.
Have you looked in the FAQ yet?
How about the Wiki?
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