I created a pr to fix non zero value in state_volume_muted

[dogruis]

https://github.com/dciabrin/ngdevkit/pull/160/changes#r3724125332

At boot time state_volume_muted should be set to zero.

I xor the accumulator a which clears any flag then store the value of a into state_volume_muted.
Found this out by creating a pause menu in my game where the sfx and the music volume can be adjusted.

[dogruis]

Pretty happy with my pause menu - might be worth an ngdevkit example. The menu is drawn on the fix layer and to dim the world behind it I just swap the whole 256 palettes for a darker array of colours. Snaps right back when you unpause because there is no transparency.