Adding functionality to existing module (eg. Audio)

Hi

I'm still new to Drupal and am struggling to understand some aspects of the concept.

What is the easiest way to add functionality/fields to an existing module?

For example:
I have the audio module and want to add some custom fields and a rating mechanism to it. We already started to recode the audio module and make a new module based on it.

Is there an easier/better way to do this? I looked into CCK but it seems only to allow creation of new node types and not to extend existing node types.

Any help and hints would be highly appreciated :o)