Vibration - What are the Main Sources?

A current thread discussing the best tweaks gave consistently high ranking for component isolation. I am curious to know where all the vibration is coming from that we are addressing with isolation. I understand that high volume listening can create significant vibration, but for the sake of this discussion let's assume we are listening at moderate levels. Can the vibrations from moderate sound levels affect the quality of sound? Are there other common significant sources of vibration that we are guarding against that can dramatically affect sound?