As for the mp3 file, I can offer you mpg123 .net wrapper that is extremely easy to use - you will get the sample for reading the mp3, extracting PCM information from it (this is main part of WAV file - PCM data for the sound).
Please, go here:
http://sourceforge.net/projects/mpg123net/
and download sample code from here:
http://sourceforge.net/projects/mpg123net/files/
Ping me if you need more info/help on the subject.
As aac files are concerned, there is faad project here:
http://www.audiocoding.com/faad2.html
that enables you to do the same with the aac file. If you need .net wrapper, I'm about to put it on sourceforge also.