How to convert mono to stereo (and stereo to mono)

The classic symptom is unmistakable: you play a recording and the sound comes out of one earbud only. That's a mono track sitting in one channel of a stereo file, and converting it properly puts the sound back in both ears.

Mono vs stereo, briefly

A mono file has one channel of audio, played identically in both speakers. A stereo file has two independent channels, which is what lets instruments sit left or right. Problems appear when the two get mixed up — a mono recording written into just the left channel of a stereo file plays silent on the right.

Convert mono to stereo

Does converting to stereo make it sound wider?

No — and it's worth being clear about this, because it's the most common misunderstanding. Converting mono to stereo duplicates one channel into two. The file is now stereo, and it plays correctly in both ears, but there's no extra spatial information to recover; true stereo width has to be captured at recording time or created deliberately in a mixing tool. What you're fixing here is a playback problem, not adding a sense of space.

When to go the other way — stereo to mono

• Voice recordings, podcasts and interviews: mono halves the file size with no audible loss.
• Audio destined for a single speaker — phone playback, PA systems, some smart speakers.
• Fixing a recording where one channel is clean and the other is noisy or empty.

Fixing one-sided audio in a video

If a video plays in one ear only, extract the audio with Extract Audio, convert it to stereo, then re-attach it using Add Music to Video in Replace mode. The picture is untouched.

Once it's converted, Normalize Audio is worth a pass if the recording is also quiet, and Compress Audio will shrink the file for sharing.