Every so often, a problem arises with the footage you are editing. whether the problem is low audio, missing dialogue, or a jump cut, every editor knows a trick or two that will save them in a pinch. If done correctly, the viewing audience will never even know it was wrong to begin with. If you find that you've turned a clip's audio level as high as it will go and your audio is still too quiet, there is something else you can try. By copying the audio clip and pasting it directly in the audio track below, you can increase the loudness of the sound.
Even consider locking the horizontal movement of your clips so that there's no chance the audio will shift forward or backward. This will allow you to localize which clips you are boosting since you are refraining from using a global audio gain. You can continue pasting copies until it's at an audible level. However, keep in mind the more you duplicate the clip, the louder every sound will be. So if you have a slight hum in the audio, it could become quite obvious.
Be sure to also ensure you have enough audio tracks available and stay organized, you may want to group or link the audio clips together once they've reached an audible level, otherwise shifting 10 or so audio clips with one piece of video will be difficult. Now you can save that crucial interview or help a quiet actor through your audio editing. There is a trick for almost anything. If you pay close attention to movies and television shows you'll see that even the professionals use editing tricks.
By learning the tricks already out there, and stumbling upon some of your own, you'll find that challenges in the editing room won't be so difficult to overcome.
To learn more editing tricks check out Editing Dirty Little Tricks.