People will tolerate an imperfect picture longer than unclear sound. The fastest audio upgrade is not a new microphone—it is reducing echo and moving the existing microphone closer.
Choose the softest room
Curtains, clothes, rugs and furniture absorb reflections. Avoid empty kitchens and bare walls. Clap once: if you hear a metallic tail, move or add soft materials outside the frame.
Bring the microphone closer
Place the phone 50–80 cm away when using its built-in microphone. A simple wired or wireless lavalier should sit around a hand span below the mouth and away from rubbing fabric.
Record ten seconds before the take
Listen through headphones for hum, traffic, air conditioning and clipping. Ten seconds of checking prevents an entire unusable recording.