Just had an unfortunate experience. Connected Bluetooth earbuds for the first time and the GUI sound FX were painfully loud, so I went into audio settings and changed the volume down to 50%. Much better.
I was then navigating around and suddenly got deafened again by a GUI sound. Genuinely painful. Anyway, this led me back to the audio settings where I was surprised to see it back at 100%.
A bit of investigation later - it turns out that all 3 settings (Rear Audio Socket, HDMI Audio and, if present, Bluetooth devices) reset back to 100% after a bit of navigation.
To reproduce: set one of the volume sliders to 50%, click on Games and Applications, go back to Audio settings (it should still show as 50%), click back on Games and Applications, and then back to Audio settings again - the volume will be back at 100%.
