Convert Aac to mp3 in C#

I would highly recommend the open-source FFmpeg or libavcodec library: http://en.wikipedia.org/wiki/Ffmpeg[^],
http://ffmpeg.org/[^],
http://en.wikipedia.org/wiki/Libavcodec[^],
http://libav.org/[^].

In both cases, you can get free open-source utilities which can make nearly everything, including conversion of the video files you mentioned, which I actually used on regular basis.

Now, how to use it programmatically in a .NET application? First, you can use the available utility by running it using System.Diagnostics.Process.Start:
http://msdn.microsoft.com/en-us/library/system.diagnostics.process.start.aspx[^].

If this is not good enough, you can wrap the library in a .NET assembly by yourself or find appropriate wrapper. Please see:
http://www.ffmpeg-csharp.com/[^],
http://sourceforge.net/projects/sharpffmpeg/[^],
http://vbffmpegwrapper.codeplex.com/[^].

After all, try to find some more: http://bit.ly/VpboUJ[^].

If you wish to work at such wrapper by yourself but don't know how, ask a question, I'll give you the basic ideas (using P/Invoke or C++/CLI "mixed-mode" project).

Good luck,

—SA