You basically want to volume level automation raise up a few dBs during song, then go back down to previous level after the little part in question. There are numerous ways to accomplish this. Here's one:
On the singer's voice track, change automation mode (at bottom of the track lane) from "Auto: Off" to "Write", so that now your volume adjustments will be written to the singer's track
Make sure cursor is before the little part in question, then start playback. When at the part in question, slide the volume slider up a bit. When past the part in question, move the volume slider back down. Stop playback. Set the track's automation mode to "Read" (So that your volume adjustments will be read from the singer's track) and now the volume slide will make the same adjustment every time you play the track.
If you want to adjust the volume change, press 'A' to see the track automations and then you can make edits with your mouse. Alternately, if you are not happy with your volume change, re-engage "Write" mode again and re-try the volume adjustments during playback. Write mode will overwrite your previous adjustments, so it's just like starting over. (If you REALLY want to start over, you can use Undo history to skip back before your original "Write" mode changes.
