Save the soundtrack of your own footage — a voiceover, an interview, a live take — as M4A or WAV, and reuse it as the music track in your next edit.
M4A (AAC) is small and plays everywhere. WAV is uncompressed — larger, but ideal if you'll edit the audio further.
Only extract audio you own or have permission to use. Music in other creators' videos is usually copyrighted.
No. The audio is decoded and encoded on your device.