Notes to myself!

Since many audio editors (except Sound Forge and other paid programs) don't support importing/exporting AC3, luckily the awesome FREE Audacity now does. Welcome to Audacity Audacity® is free, open source, cross-platform audio software for multi-track recording and editing. Audacity is available for Windows®, Mac®, GNU/Linux® and other operating systems. Check our feature list, Wiki and Forum. Download Audacity 2.1.3 Mar 17th, 2017: Audacity.

Suppose you want to create a surround sound file using Audacity. The app is a little obtuse, so this may clear up some confusion.

When exporting, Audacity defaults to mixing down to stereo. You must adjust these settings:
Edit → Preferences → Import/Export → Use Custom Mix

Lay out your audio. Keep each track as mono. You can have as many tracks as you like and then downmix them later.

File → Export
If you choose a surround compatible format, like .ogg, you will be able to assign each track (on the left) to an output channel on the right.

Audacity Ac3 5.1

The channels don’t have names – which is really unhelpful. Here’s how they map according to the OGG Specification.

Download | Audacity

If you want to test it for yourself, here’s a simple multitrack ogg.