Media Average Volume for Normalization

We want a class or function that receives the path to a media file and return the average volume of the file for purposes of normalization. Something like this: double getAverageDb (const char * a_fileName); Restrictions: The function or class can be used in other libraries availables in Linux (preferably Ubuntu) as GStreamer. The time for execution of the routine should be preferably less than 1 / 4 of the normal runtime of the media. Supported formats must be at least mp3, avi, mpg, but preferably each supported by GStreamer. P.S. We will use Qt 4.7.2, so you can use too if you find something to help you.