A comprehensive production music library, like Iamusic.com, can be an excellent source for music. Music Libraries typically offer large selections of music and styles to choose from, whether they are a royalty free music library or not. With the right music library, you can find all the tracks you need, and get all the licenses you need from one source.
The music is already finished (unlike custom made music), so you know EXACTLY what you are getting BEFORE you license the music.
This option's greatest strength is the ability to have music created specifically for you, and to acquire exclusive rights to its use.
The biggest consideration here is the leap of faith required on your part. You have to 'trust' the final product will be finished on time, and be exactly what you had hoped you were going to get. Unlike the cost effectiveness of music obtained through a music library, custom made music can be 40 to 200 times greater in cost per song.
You will want to seek out a professional composer who can create music authentically in any style you request. Unfortunately, the majority of composers are only well versed in one or two styles. When choosing the option of custom made music, give yourself plenty of time between the due date for the delivery of music and the release date of your project. Have a back up plan in case the music is not delivered on time, or is not what you expected.
Request to hear 'demos' of the music before it goes to the final recording phase. Be prepared to pay the costs to produce these demos. Demo costs are typically nonrefundable, but you should ask for these costs to be deducted from the total license fee if you want the composer to proceed with the recording.
MIDI stands for "Musical Instrument Digital Interface". The most compelling reason to consider using MIDI files are their small footprint. You can also change how loud individual sounds are, change the speed of the song, and remove any unwanted sounds or instruments.
The biggest problem is a lack of sources to obtain MIDI files (which will not get you into copyright troubles), and the final sound of your project will be totally dependent upon the end users sound card or other MIDI device.
MIDI files contain no audio data, but instead merely provide instructions for a general MIDI device to reproduce the music using the sounds it has available, such as a sound card. While most MIDI files are created using $30,000 MIDI rigs, this means nothing when your customer hears the music in your creation through their $19.95 sound card.
If you are a manufacturer of a General MIDI (GM) Sound Card or GM Tone Generator, MIDI files can really showcase the awesome sounds and capabilities of your product. You have the advantage of knowing how the MIDI files will sound to your end user because they will be playing them with your product.