Apple - Convert an already recorded stereo track to mono in Garageband

This was driving me mad, but I now use an very smooth way based on JHerran's answer. It has the virtue of only requiring Garageband and is quick.

N.B. I assume your track has a single region: if not you will have to shift-click to select all the regions in the track that you want to convert.

1. Click on the track that you want to convert. The single region should automatically be highlighted.
2. Command-D or >Track>New Track with duplicate settings to create a new track
3. Command-V to paste the original regions to the new track
4. Shift-Click on the original region(s) to highlight it again
5. Command-J to join the regions, select "Mono" for the mix-down
6. The mix-down will replace the original stereo track - you can now click on the temporary track and Delete it.

There's a command line approach to this that, once you get it all set up, is a little more straightforward.

First you need to install Homebrew, the best package manager for OS X out there. Open up Terminal and type:

ruby -e "$(curl -fsSL https://raw.githubusercontent.com/Homebrew/install/master/install)"

With Homebrew installed you can install all kinds of cool command line tools using the brew command. We're going to use it to install sox, "the Swiss Army knife of sound processing programs":

brew install sox

With sox installed we can split our audio file in to two mono tracks with one simple command call. First, from GarageBand, export your stereo track to disk as a wav file. With the file on disk, open up a Terminal program and change to the location:

cd ~/Music/GarageBand/bounces/

And then run this sox to command to split the file in to two mono tracks, one for the left channel and another for the right channel:

sox My\ Song.wav My\ Song.L.wav remix 1
sox My\ Song.wav My\ Song.R.wav remix 2

sox will keep the WAV settings the same as the source material so you should be able to, quite easily, drag and drop these split files back in to GarageBand as mono tracks.

2017: it’s in the Track controls (where the sends and reverb and stuff are, double-click on the track header) next to where it says “No Input” (or possibly input, depending on what you have set up). There are two intersecting circles. Click that and it becomes one circle. This works even AFTER recording, I just checked. Hope this helps! I made a little illustration because this took me WAY too long to find just now, even with the help of this thread. There are too many things in garageband that are buttons that do not look like buttons.