Galaxy Tab A SM-T280 sound problem

[Earthrise]

My daughter has a Galaxy Tab A SM-T280 running Android 5.1.1. It has developed an odd sound problem that I can't find any help with elsewhere.

The speaker works just fine. The problem is that almost all sound through headphones sounds distorted - the best description is it sounds like you are underwater. I have tested it with the Youtube app and website, VLC Mediaplayer, the Android Video app and the BBC website. All give the same weird sound.

The only time I can get the sound to sound normal is if I go into settings and use Adapt Sound. All the sounds I heard when I tried to set that up were completely normal. I have tried Safe Mode and there was no change. Haven't done a factory reset yet as they mean a lot of work setting everything up again and I don't know if one will even help.

Does anyone have any ideas what's going on?

[AndrewL]

@Earthrise: Do you have access to another set of headphones that you can try to see if the problem persists? If you head to Settings > Sounds and notifications > Sound quality and effects, are SoundAlive+ and Tube Amp toggled on? If so, and if you disable these features, does the sound quality improve? Furthermore, tap the Back button, select Volume, and move the Media slider further to the left to reduce the overall volume output for Media apps. To address any temporary software issues, please try restarting the tablet to see if you notice an improvement. Let me know how you get on. 

[Earthrise]

Happens on all pairs of headphones.

SoundAlive + and Tube Amp are off.

Changing the volume has no effect.

Restarting has no effect.