There are two main difficulties in capturing Podcast Soundboard audio in GarageBand: for one, GarageBand cannot capture audio directly from other software, and secondly, GarageBand can only accept a single audio input source. For these reasons, a third-party tool called Soundflower must be used to create a virtual audio input device, and an Aggregate Device must be created so GarageBand can hear both your USB microphone and Podcast Soundboard at once.
Podcast Soundboard is configured once Soundflower is installed and you've selected one of its virtual audio devices as the Podcast Soundboard's output. See the Installing Soundflower section of this article to get Soundflower installed.
Next, we'll configure a GarageBand project that can record Podcast Soundboard audio but nothing else:
With these options selected, you should be able to record audio from Podcast Soundboard.
The next thing we need to do is create an Aggregate Device so that our microphone is captured as well. See the Setting Up An Aggregate Device section of this article. There are nuances here, though! I've found that the microphone has to have a two-channel format, like this:
So that the aggregate device can consist of two mic input channels and two Soundflower input channels, like this:
I've found that GarageBand struggles with aggregate devices with more than four input channels (this may just be me).
Now, select the new aggregate device as your input device in GarageBand preferences.
There are more steps, but right now you should have one input track which is recording channels 1+2 of your aggregate device (the microphone):
The next step is to add another track by hitting this button:
But this time we want to capture channels 3+4:
We're nearly done! We just need to arm both tracks to record at once. This involves right-clicking on a track and selecting Configure Track Header...
Then hit Record Enable:
You probably want the second track (input channels 3+4/Podcast Soundboard audio from from Soundflower) to have input monitoring enabled. Then, with both channels armed to record, you're ready to go. Your setup should look like this, where the Record Enable buttons are blinking:
And finally, here's some evidence of the whole thing in action:
It's always a pain to record multiple input sources like this. Thankfully, Soundflower and aggregate devices are convenient tools that make it possible.