Vanilla Music

[Smeelio]

I know there is mods for it, but if you put a song into the .minecraft folder that holds the music, would it play?

[EzioA]

No, you need special mods for that (e.g. SoundMod by Risugami, it creates mod sound folders and you can put .ogg files in there and it plays it like the normal MC music)

[Industrial Miner]

I prefer KBI More Music Discs for this. You can add as many discs as you want and craft them all too. You will need to make the shapeless recipe for custom discs in the config file.