Open an online speaker test that plays tones to each channel individually. You should hear sound from only the left speaker/ear, then only the right. If one side is quieter or silent: on Windows go to Settings > Sound > click your output device > check the Left/Right balance slider and center it. On Mac, go to System Settings > Sound > Output and center the Balance slider. For headphones, try a different pair to rule out a broken driver. For wired speakers, check that both speaker cables are fully plugged in and the speaker wire isn't damaged. If balance is off in only one app, that app may have its own balance control (check audio settings within the app). Mono audio mode in accessibility settings will combine both channels into one — turn this off if you want true stereo.