It's taken me awhile, but I've finally figured out how to find which files sysprep needs to install things like audio and video drivers.
1) Install the driver using its install program.
2) Check the device settings, note in driver details what files are installed.
3) Look in c:\windows\setupapi.log
4) Look for references to your particular driver, there should be a section reference and an inf reference. For example, for the Sapphire Radeon HD 3650, look for [ati2mtag_RV3650] and CX_70226.inf
5) Scan the inf carefully to figure out what files need to be copied to sysprep. Also look for something similar to [SourceDiskNames...] at the end of the file. Sometimes if you have drivers that are in sysprep but still cannot be found by sysprep this is the problem.
6) Test on a clean image. Repeat.