SmartSound.com is a royalty-free audio library featuring tracks that can be customized with length, instruments and intensity using its proprietary Sonicfire Pro software. It’s pretty powerful stuff that offers timing control, mood mapping, custom length, variations on the tracks and the ability to search all of the tracks directly and make purchases without leaving the software application.
I’m a big fan and frequent user of Smartsound. All of the music from my recent short Nerfed and my intro segments from my CES videos were built inside Sonicfire Pro 5.8.3 and, ultimately, ended up in my final sequences in FCP X. Now, the company has extended the power of Sonicfire Pro with a plugin for Final Cut Pro X, which allows you to use a generator to create a custom length audio track and then select a track from Sonicfire Pro to send to a library within FCP X.