I need to be able to take an mp3 and extract only channel a from that recording.
You want to keep only the left or right channel, right? A stereo MP3 has left & right channels, is that what you mean by A & B channels?
There is a little drop-down arrow to the left of the waveform. Click that and select Split Stereo Track. Now you should see the tracks labeled Left & Right, each with their own set of buttons & controls to the left of the waveform.
Next to the drop-down arrow you should see the file name, and to the left of that you should see an X. Click on the X to delete that track.
On the remaining track, click the drop-down arrow and select Mono. (I’m assuming you want a mono track that plays from both speakers.)
Now go to File-Export to save-as MP3 (or the format of your choice). If you want to make a new MP3, you’ll need to install the optional LAME MP3 encoder, if yo have not done so already.
NOTE - When you edit an MP3 (with any “normal” audio editor) it has to be decompressed first. MP3 is lossy compression, so if you save as MP3 (or AAC, etc.) you are going through a 2nd lossy compression step. You may might not notice the quality loss and you may not have a choice anyway, but its something you should be aware of… If you want MP3, ideally you should compress to MP3 ONCE as the final step.