5.1

Assuming you're wanting to convert to Dolby Digital 5.1 you'll need a "real" AC3 encoder. AFAIK the cheapest way to get one is to buy Sony Vegas for around $500. Some versions of BeSweet (a free audio tool) have AC3 encoding functionality that may work, but it's buggy and last I knew had been removed from that program.

There's a guide at Doom9 with a lot of information (if you decide you want to spend the money to do it).
http://forum.doom9.org/showthread.php?t=56020

Edit: I just noticed you posted the same question yesterday. Let's avoid multiple threads for the same question in the future.

 

It's the only one I'm aware of.

 

Yes, but what you need to do depends on your player. With some players (I have no idea how many but I imagine it's a small number) you just have to make a data disc with the file on it.

A more universal method would be to author a simple DVD-Video disc with nothing but a black frame for video and the AC3 track for audio. This can all be done relatively simply with free tools, although some people would argue about my idea of simple. I'd use AviSynth's BlankClip filter to create the smallest DVD compliant video frame possible. The exact settings for the filter would depend on whether you're making a PAL or NTSC DVD and how long the audio stream is. Then I'd use TMPGEnc to encode the video to MPEG-1. Then I'd use some free authoring software (probably Rejig) to author a DVD.

I know I've posted more specific instructions for this a couple of times in the past, but a forum search isn't giving me anything useful and it's been a pretty long time ago.