LineIn is a simple application for OS X to enable the soft playthru of
audio from input devices. In simpler terms, you can use LineIn to play
sound coming in through a microphone or any other device plugged in to
your Sound In port, just as was once available with OS 9's "Play input
through output device" option.
See Also: [SoundSource][1]
Compatibility
Architecture: PPC x86 (Intel:Mac)
Version 230 is for Lion - Sierra (10.7 - 10.12)
Version 211 is for Snow Leopard (10.6)
Version 203 is for Tiger (10.4)
Version 202 is for Panther (10.3)
Version 120 is for 10.1 & 10.2
[1]:
http://macintoshgarden.org/apps/soundsource