System sounds but no other sounds when use headphones

I plugged in my headphones this morning, which were working fine yesterday, and went to try and watch a video only for there to be no sound. But all notifications and click events seem to be making sound as usual. And only certain apps seem to be not making sound. Thinking it was possibly a bug in YouTube, I tried running other apps: - Spotify: no sound - Google calendar notification :**full sound** - Hearthstone, a game : no sound - Analogue Synthesiser emulator: **full sound** - ZType, a game: **full sound** - Shadowrun returns, a game: no sound - BBC iPlayer: no sound - Sony Xperia Podcast app: no sound - Pointless, a game: no sound. I've yet to test with other headphones. But have tried a restart and tried double checking the volume control advanced settings. Any idea what the reason is? If it was all sounds or only loud sounds it would be simple, but it's not, which is what confuses me. *further testing* After I got to a desktop PC did some further tests: - tested same sources with a different set of headphones, same result. - tested the original set on the desktop PC, produce sound from all sources no problem. - Set phone to safe mode to see if it would be an app which has a damaging bug after update (as I've set my phone to update when on wifi and charging, so it is plausible) the behaviour was the same.