In the sample ALAC file attached, converted using:

  avconv -i input.mp4 -acodec alac -t 20 output.m4a

The audio bitrate appears as 0kbps. Nautilus shows it as 730kbps, MediaInfo shows it as 713kbps, and avconv shows it as 731kbps.

Code used:
hyperair@thinkpwn:~/tmp% dmcs taglib-show-bitrate.cs -pkg:taglib-sharp
hyperair@thinkpwn:~/tmp% ./taglib-show-bitrate.exe David_Guetta_-_Titanium_ft__Sia-JRfuAukYTKg.m4a
Bitrate = 0
hyperair@thinkpwn:~/tmp% cat taglib-show-bitrate.cs
using TagLib;
using System;

public class ShowBitrate
{   
    public static void Main (string[] args)
    {   
        TagLib.File file = TagLib.File.Create (args[0]);
        Console.Out.WriteLine ("Bitrate = {0}", file.Properties.AudioBitrate);
    }
}