Most of the music in videos created prior to early 2008 comes from either Sony Cinescore or SonicFire by SmartSound.
I have since mostly abandoned those tools in favor of creating my own music (well, sort of) with Sony ACID Pro 6. It's a lot more fun but not nearly as technically demanding as creating compositions truly from scratch via MIDI, and vastly easier than trying to do anything acoustically (which I am not really good enough to do anyway).
I have created some artificial sounds with Mathematica, using sine waves at different frequencies with various amplitude and frequency modulation envelopes.
Some other individuals have contributed music as well, as described in the credits for specific videos. They retain copyrights to their work and in all cases they have given me permission to use it. It is much appreciated.