Music is the Soul of your film.
Make no mistake, if you enjoyed the film, its probably because of the soundtrack.

Music, and I will include Foley and sound design here too, is so fundamental to your film that it is the perfect twin to the necessary needs of having to have a camera so your audience has something to watch.
Your audience will see your film, but they will feel your soundtrack;
Your music choices, Foley, sound design, diegetic & metadigetic sound add that attention to detail that will inform your audience of where they are in the environment of your film.

A great soundtrack, whether is from a CD off the shelf, a Creative Commons website or an original, hand crafted track is so fundamentally important to your films appeal and connection to your audience that you really should go back and watch your favourite films all over again, but listen to them this time.

Great sound design (an engine roars, keys in a lock, the breath of the killer and the panting of their victim, pew-pew goes the laser gun) are all carefully considered, carefully made decisions. Vision is Kinetic, but Sound is Visceral

Listen to Wall-E, The Aviator, The Life of Pi and Star Wars IV to see what is meant by great sound design.