Macbook 2011 Audio Input/Output Question
On my iMac (21.5-inch, Mid 2011), I have to open QuickTime Player and select File-&gt;New Auto Recording from the menu bar. This will produce the Auto Recording window shown below. 



You should make sure you have Line In selected, as shown below.



You should also set the Output volume and the Line in Input volume in the Sound pane of System Preferences, as shown below.



Also, you should also set the Headphones Balance in the Sound pane of System Preferences, as shown below.



You should be able to hear the sound from the input jack. You should be able to do this without actually recording the sound.

The version of macOS was High Sierra 10.13.6.