There's no denying that a carefully crafted music track can help underscore dramatic moments and provide a supportive backdrop to a complex piece of storytelling, adding both a sense of continuity and necessary punctuation to a film.
The key, of course, is to understand how music can support a thematic mood or dramatic idea and then choose tracks that reinforce that modality. Cutting the material to accurately match on-screen images might still require a music editor, whose musical sensibilities and understanding of tempos and structure will let the score flow more seamlessly; an experienced picture editor can also quickly pick up the basics and offer a viable alternative.
But you need to realize that these libraries aren't free; before you use any track, you need to license and pay for it. And no library carries well-known songs by famous artists or—with notable exceptions—offers exclusive use of any cut. However, most libraries offer cuts that are "style-alikes," meaning that they have the same style and sound as a commercial cue you might be looking for. In addition to style-alikes, most of the larger libraries also will create music on-demand. While this function may increase your costs, it's a useful solution if you can't find what you're after elsewhere. (Some libraries have access to an in-house recording studio and roster of artists associated with the company.)
Due to an increase in products needing music for the Internet and other digital media, there currently exists a large number of high-quality library sources, which fall into two basic types:
1. Large production music libraries, which often represent a number of smaller collections, offer virtually unlimited styles and genres. Knowledgeable staff often are available to help you find what you want; since these are full-service, one-stop operations and own all of the musical rights to their music, it's a simple process to license a cut for a series of targeted uses.
2. Independent libraries concentrate on representing artists with whom they have licensing agreements and for whom they act as the publishers; their focus is song placement in film and TV, which these days is a growing trend. Although they're often not as large or diverse as the bigger libraries, independents offer a variety of music and can often satisfy your musical needs, with helpful staff to get you through selection and licensing.
Also look for royalty-free music libraries that sell cuts that are yours to use forever with minimal restrictions.
Most of the music available from both larger and independent libraries will include an underscore and cut-down versions of each cue. This means that you'll have available—in addition to the long cue—versions that are precut to 60, 30 and 15 seconds, as well as a version that excludes the main melodic material. In this way, for example, you can intercut the fully produced cue with the underscore and place it under narrative so it won't collide with the voice.
Most libraries provide a website with powerful search engines that let you find, audition, choose, pay for and download your music, usually in WAV or AIFF formats at 44.1/48 kHz sample rates or 128-320 kB/sec. MP3-format files directly compatible with most popular audio workstations. Should you need another format or music version, a phone call or email often will produce the desired result.