From the Logic Pro 9 User Manual:
[i][b]Using Auto Input Monitoring for Recording[/b]
If Auto Input Monitoring is turned on, you will hear the input signal only during the actual recording--before and afterwards you'll hear the previously recorded audio on the track, while Logic Pro is running. This helps you to judge punch-in and punch-out points when you are punch recording. If Auto Input Monitoring is turned off, you will always hear the input signal.
[b]To turn on auto input monitoring[/b]
Do one of the following:
Choose Options > Audio > Auto Input Monitoring from the main menu bar (or use the Toggle Auto Input Monitoring key command).
Control-click (or right-click) the Record button in the Transport bar, and choose the Auto Input Monitoring setting from the pop-up menu.
Your individual recording situations will determine whether or not to use auto input monitoring.
Consider the following examples:
Singers cannot hear themselves while Logic Pro is in playback mode---all they can hear is the old recording. In this case, turn off Auto Input Monitoring.
The recording cannot be heard in playback mode. In this case, turn on Auto Input Monitoring to hear audio on the track.[/i]