Much of Adobe Soundbooth CS3's audio-only functionality can be found in less-expensive audio editors. One feature that breaks with these editors is Adobe Soundbooth CS3's AutoCompose Score. Invoke this command, choose a Soundbooth score template, and the program will create a background music score that matches the length of your video (or a duration you've set within the AutoCompose Score tab).
Once you've created the score you can change some of its characteristics. For example, within a Basic editing setup, an Intensity slider adjusts the emotional impact of the score by adding or reducing instruments (a screaming guitar lead that sounds when you drag the slider to the right, for example, adds to the intensity). You can also add or reduce melody instruments by dragging Melody sliders. Drag a Score Volume slider in the Basic editing setup to change the score's overall volume.
For more control, click the Keyframing button and you have the ability to adjust Intensity, Melody, and Volume characteristics by adding keyframes. Just move the playhead to a point where you'd like to add a keyframe, click the Add Keyframe button within the Intensity, Melody, or Volume lanes, and adjust its level (either by dragging it up or down or using the numeric percentage control). Soundbooth will create transitions that make musical sense between areas where you've changed one of the parameters. Once you've created a score you can export it as a separate AIFF or WAVE audio file or incorporated in your movie as a QuickTime .mov file.
The demonstration scores provide a glimpse of how the AutoCompose Score function works, but they're not terribly impressive when you want to score a video longer than about 30 seconds. Unlike scores created with such dedicated music generating applications as SmartSound's SonicFire Pro, the scores don't offer much variation. Adobe Soundbooth CS3's longer generated scores get repetitive in a hurry. It's my hope that the final version of Soundbooth will offer score templates that can stand up to longer-form videos.