Sound Siphon is a software package that makes your Mac’s audio output available as an audio input device. It enables you to send audio from one application to another where it can be processed, streamed, or recorded. It's as simple as selecting a microphone in your favorite app. Sound Siphon works with Core Audio to provide this capability to all of your audio apps. As such, it easily integrates into any existing audio production environment. Features:

• Capture all of your Mac's audio
• Capture specific application audio
• Independently adjust the levels of each application
• Capture more...

Version 1.1.0:

• Changing the play through device in preferences works more reliably.
• Unregistered effect has been changed to a bandpass that is turned on and off periodically.
• Now works on Yosemite.
• Added simple on/off switch to main UI.
• Fixed a bug that would result in hearing now audio on a clean install.
• Uninstaller is now included on the installation disk more...

Requirements

• OS X 10.8.4 or later