Audio is 50% of video quality. Viewers forgive bad video but not bad audio.
Recording
Use an external microphone (even a $20 lav mic beats built-in). Record in the quietest location available. Monitor audio with headphones while recording. Keep mic close to speaker (12-18 inches max).
Post-Production
1. Remove background noise. 2. Normalize volume to -16 LUFS. 3. Add compression (even out loud/quiet parts). 4. Trim dead air. 5. Export as WAV (not MP3) if your video editor supports it.
Music Mixing
Voice: -12dB to -6dB peak. Background music: -25dB to -20dB. Sound effects: match the perceived loudness of the scene.