I can't record audio from app X (your app name here)

What gives?

If you're recording Skype; but not "hearing the other person", or recording some app and that app's audio isn't coming through, then keep reading.

Basic idea

There are some apps that:

1. Don't play well on Mac (they are not good citizens)
2. Specifically ignore changes to the default sound output

iShowU uses the SWB Audio Capture driver to capture audio. For this to work, a couple of things must happen:

1. The drive needs to be installed
2. The app being recorded needs to output it's audio to the driver.

#1 is easy. Click the install button. Enter your password. Done.

Normally, #2 happens automatically, but some apps (Zoom (sometimes), GotoMeeting, Vimeo, Facetime) don't.

When iShowU changes the Sound | Output to SWB Audio Capture (see that here), these apps do not see the change. They continue playing audio to their last device (probably the speakers). Net result? You hear it, but it isn't recorded.

Solutions

1. See if the app can output it's audio to the "default device".  Here's that setting, in Skype (this is just an example, and likely different in your app, but see if the same idea applies):
2. Output directly to the SWB Audio Capture driver. Here's a pic from Skype. Notice I'm about to change it to SWB Audio Capture.

But I don't hear anything!

Some ideas:

1. If you use option (2) above, it's possible you don't have monitoring turned on in iShowU. That would mean iShowU would be capturing the audio, but not playing it back anywhere. 
2. With iShowU Instant, there is a "Audio - Preview Device" preference.  It's in the app's preferences, under General. Try changing that to a specific device (such as speakers, headphones, etc).