People will be able to stand some bad visuals but if you have bad audio it will kill your film. We find it grating, and annoying to hear bad quality sound this clip is only a minute long but the bad sound quality is makes it almost unbearable.
Author Sara Caldwell explains why audio is so important.
Audio can be very finicky and, take a lot of adjusting to make it sound just right, but a good rule of thumb is that when shooting your natural sound it should fall just above -12db and for interviews and voice overs it should be about -8db to -6db. Now these are general rules and can vary from camera to camera, and mic to mic.
Speaking of cameras and mics; should you use cameras or external mics to capture sound? Well that depends first you need to think about what type of mic will suit your needs, and then you want to think about your resources ,like if your doing it solo, or with a crew.
Personally I would use a wireless external mic because of the versatility it offers while also keeping you equipment load low .
The best way to monitor your levels while filming is to wear headphones.
A mistake a lot of beginners will make is they will just use the levels on their camera or on their audio recorder to judge the level but the forget it is a human ear that will here the sound so a human ear is going to be the best judge of sound volume and clarity.
This site offers some good tips on getting good audio for a home movie but they also work for documentary films.
So remember keep the audio levels for natural sound around -12dbs, and interviews between -8dbs and -6dbs, and always wear headphones to judge the sound quality.