HTML5 Audio - Not displayling length of track

I'm having some problems regarding HTML5 and audio.

The markup is standard and minimalistic:

<audio controls>
    <source src="deliverAudio.cgi?test.mp3">
    <em>Meh, your browser does not support HTML5 audio</em>
</audio>

As you can see this is completely minimalistic, the only quirk is, that the audio content is delivered via a cgi script.

In fact this works, the audio is played, but the player does not show the complete length of the track, instead it displays the amount already played and you can only navigate in already played portions.

This behaviour applies to Firefox 31 and Internet Explorer 11, I have not tested with Chrome, but I highly expect it to be the same.

The cgi is very minimalistic too, it reads the audio file in binary mode and prints it out.

I really don't know where to go now, because it actually plays, but is not displaying the length.

Any help and useful comments are highly appreciated!

Thanks in advance!