I've started work on the Media Transcriptions module today. This module will allow the attachment of Transcriptions (or Closed Captioning) to video and audio files. The initial version in CVS is from some work by Jonathan DeLaigle (grndlvl), who is a co-maintainer & developer of the module.
For full functionality, you'll need to wait for Drupal 7, because of the long-awaited hook_file and Fields in core issues.
However, there will be releases with limited functionality for Drupal 6, and even a teaser for Drupal 5, with a promise of smooth upgrading as things progress.
Read more to Find out What this Means for your Multimedia!