Why do my headphones only play in one ear?

The cause is almost always identifiable with two quick tests, and it is frequently not the headphones.

Test one: try them on a different device. If the fault follows the headphones, it is them. If it stays with the device, it is a setting or the socket.

Test two: try a different pair on the original device.

If it is a setting — and this catches people out surprisingly often:

Audio balance slid to one side. Both Windows and macOS have a left–right balance control, and accessibility settings on phones do too. It can be changed accidentally and is rarely looked at.

Mono audio enabled in accessibility settings, which should play the same in both ears but interacts oddly with some hardware.

The source recording is genuinely one-sided. Some older recordings, and some poorly encoded files, have content only in one channel.

If it is the headphones:

Cable damage at the strain points. With wired headphones the overwhelming majority of failures are at the plug or where the cable enters the earpiece, caused by repeated bending. Wiggling the cable near the plug while audio plays usually confirms it — intermittent sound means a broken conductor.

A dirty socket. Pocket lint compacts in a headphone jack and prevents the plug seating fully. A plug not pushed all the way in commonly produces one channel plus distortion. Clean carefully with a dry wooden cocktail stick, with the device off.

Blocked mesh on earbuds. Earwax and debris block the sound outlet, which sounds like a dead side but is a muffled one. Clean gently with a dry soft brush — never with liquid or anything sharp.

For wireless earbuds specifically:

One bud not charged, or not seated properly in the case so it never charged.

Pairing lost between the buds. Many designs have one primary bud relaying to the other; if that link drops, one goes silent. Resetting the pair — usually placing both in the case and holding the button — resolves most of these.