Idnaf.AC3ngine is an open source AC3 to MP3 converter. It is actually like an automated tool which use lame, normalize and azid. It also support multi files conversion. Yet, this application is like an automated GUI wrapper for conversion of AC3 to MP3.
This application was completely written in C# (Visual Studio 2005) and you will need .NET framework 2.0 to run the application.
System requirement : Windows with .NET framework 2.0 installed
In the following link I uploaded also the source code + binary.