Hi Mike -
Yes, that's normal -- when record is enabled on an audio track you'll hear the live input (mic) signal when you're stopped and when you're actually recording, but not when you're just playing back. This emulates traditional Auto Input Monitoring in tape recorders, and it allows for punching in/out -- you hear the playback of previously-recorded audio on the track before the punch-in, and it automatically switches to the live audio when you punch in and are recording.
This behavior is controlled by the Auto Input Monitoring option in the Record menu -- if you turn it off, you'll be able to hear the live mic signal even during playback, but you won't be able to punch in properly with the necessary monitor source-switching of Auto Input Monitoring mode.
Alternatively, even with Auto Input Monitoring enabled (the normal configuration), you could disable the Rec button and instead enable the Input button (the little green "I" button just to the right of the Rec button in the channel strip); that would let you hear the live input signal at all times, including while playing back -- it's sometimes referred to as "rehearsal mode". Just remember to turn "I" off and "R" back on before recording/punching.