In simpler terms, you can use LineIn to play sound through a microphone or any other device plugged in to your Sound In port. If you need to input audio from an external device, recreating OS 9's "Play input through output device" option, then LineIn is perfect for you.
* Several small fixes and improvements have been made.
* LineIn's icon has also been changed.
* Bug fixes and improvements for Mac OS X 10.5 (Leopard)