iPhone 7 low audio output My iPhone 7 has really low output through the headphone jack, I’ve compared it with two other  iPhone 7 using the same app and same song. The output of built in speakers is perfect but headphone jack outputs really low sound. 

I updated the firmware through iTunes and restored my data through backup and I tried turning on/off sound check and volume limit in the Settings but nothing worked. 

I had an iPhone 7 plus and iPhone X in the past and I used same backup for all of my phones. I had the same issue with iPhone 7 plus but iPhone X had no issue with the audio output. Is there any problem with my iPhone backup or there is an issue with my phone?
Try restoring the iPhone 7 to factory settings and dont place your backup on there.

See if the issue remains because if it doesnt then the backup is the culprit.

This is how to properly determine if the issue is hardware vs software data.

If the issue remains then send them in for repair.