How do I use the MacBook Pro's audio line in?

Line in is meant for recording. What you are trying to do sounds more like using the MBP as an amp/speakers setup, this is not going to happen easily.
The only way to do it would be to run a sound recording app and set the "monitor" ON - then the incoming sound would play through any speakers you may have connected.
Before you get unhappy about your computer, think about the fact that if monitoring through the speakers was on by default, the moment you hook up a mic to the MBP a feedback would occur - so it is designed well, and works as designed.

cheers.

Hi Ymer,

Have you selected "audio line in" as your sound source in System preferences / Sound?

The other thing you will need to do is get hold of a copy of an excellent little utility produced by the people who make "Audio Hijack", called "Line-In" - available from http://mac.softpedia.com/get/Audio/LineIn.shtml or direct from the maker at http://www.rogueamoeba.com/freebies/


There is no need to use a sound recording app, if you use "Line In", though that is another option.

Cheers

Rod

What are you trying to do, record or simply use the MacBook Pro speakers?

If you are trying to use the speakers, open your Audio/MIDI setup and check the Thru box. That may allow the system to pass the sound through to the speakers. If this works you do not need any of the other software mentioned above. Normally "Thru" is off to avoid feedback during recording, as mentioned above; you'd normally use headphones to monitor if you were using a microphone.

Don't thank that option is normally available in AudioMidi Setup any more, Network 123.

Cheers

Rod

Hi. This used to be easy as the PowerPC audio had this built in. You need this:

http://rogueamoeba.com/freebies/

For some reason, the new intel Mac's can no longer do this on their own. Be Cool.