The video is available publicly at this link: https://streamrift.com/browse/video/186090.html I do hear, though, that the uploader suffered legal issues for uploading it, and that New Zealand threathens 10 years of prison for possesing it. It's not like it's a new thing to post a murder clip online (Liveleak, Bestgore, Documentingreality etc). Almost as if...
Possible he played dead. The shooter went around many times and shot the corpses to make sure they're really dead. His camera work wasn't praiseworthy, so I'm not sure he shot the 'vanishing man'. You van see the video for yourself, though, and make sure for yourself if he took the bullet or not.
And about the sound problems being due to the camera, he did listen to "Remove Kebab" on his car radio at the beggining of the video, and it sounded clear and well enough, so I don't think there should be problems with anything else, least of all the rifles.