Speech Recognition for Searching Files

Here is the problem I have:

I have a lot (tens of thousands) of mp3 files that my users would like to be able to search. Is there is software out there that you've used or heard good things about that would allow me to index that content and put it in a database so I can search on it later?

From stackoverflow

• I've heard very good reviews of Dragon Naturally Speaking, by Nuance. They offer a software development kit, but I couldn't find out any information about pricing for small projects.

  Jon : Do you know if there is a developer's API?

  Jonathan : It seems that they do, yes.

• There's an open source library Sphinx