Why It Matters
Audiences are remarkably forgiving of average video quality but remarkably intolerant of poor audio — muffled, echoey, or noisy sound will cause most viewers to scroll away within seconds, regardless of how good the content is.
Getting Audio Right
A simple clip-on or lavalier microphone dramatically improves audio quality over a phone's built-in mic, and recording in a room with soft furnishings (rather than a bare, echoey space) makes an immediate difference.
Takeaway
If you invest in just one piece of equipment this year, make it a basic external microphone — it solves the problem viewers notice fastest.