I don't know about SMB specifically, but generally music in NES games is stored in a format that somehow represents the musical score (similarly to MIDI or MOD), rather than as a waveform representation (like a WAV or MP3). So you will need to be able to transcribe or aquire the score of the songs to be added. If you are able to that, then you will need to convert the score into the representation used by the game's sound engine.

NES games have custom written sound engines and there is no standard format. While some developers reuse the same sound engine from game to game, generally game music isn't directly compatible with other games. To add music to a game, you must translate the music data into the format expected by the game's sound engine. I don't know the specifics of the format used by SMB, but it is a popular game and is likely to be well documented. Check out the documentation database here at RHDN and you'll proably come up with some useful information.