Let's Talk About Music
Music is a global language. For sure, it is much easier to listen to it than to write or even read it, but nonetheless it has the ability to provide several very important aspects to the game. Music is divided up into what are called genres, or styles.
The first thing you will establish about your game is the musical style/genre. You have a nearly unlimited range of options in regards to style- from the many styles of orchestral to 8-bit, rock, pop, and even jazz and ambient. Each genre has different limitations and requirements for the composer, or writer of the music. For instance, a genre with orchestral sounds might require you to find a suitable composer with the right ability and virtual instruments or samples to produce quality orchestral.
Remember that composing is not a universal thing, each genre has its own required skills and base understandings to produce- an 8-bit composer might have trouble writing lounge jazz, for instance, so it is important to decide your genre before you pick your composer. For the most part, it's best to let the composer have a good say in this unless this has a story purpose. Sometimes it's best to consult a composer first for ideas and advice and then go hire someone.
To do this, it is smart to try a few tracks in your game of each genre, or go listening to music online. In addition, it is in some cases okay to use multiple genres, but note that you may need to find multiple composers, or find the tracks (but more on that later in the section on sources of music).
A general rule of thumb for a good game score is to restrict yourself to one set of styles or one main genre focus. This creates a strong feeling of continuity, which allows the game score to flow smoother between scenes. In some cases, music in one area may need to clash to create contrast, but overall, one genre should be focused to represent the main feel of the game.