From King's Waste of Time:
Toggle_playthru toggles the hardware_playthru attribute of all attached sound input hardware detected on your system. This is a stop-gap utility until Apple adds the "playthrough" checkbox from MacOS
This review was originally posted on VersionTracker.com. Had no problems downloading, running, or using this app. It's free, it works like a charm, and doesn't hog up any resources like LineIn. Perfect.