How about a slightly larger button that has a bit more complex graphic: (speaker emitting sound / speaker crossed over). Ie the button has two icons with a sla

How about a slightly larger button that has a bit more complex graphic: (speaker emitting sound / speaker crossed over). Ie the button has two icons with a slash between them. One of the icons is in black outline and one is in grey outline. Pressing the button switches between what is black and what is gray. If speaker emitting sound is black, then sound is on. If crossed speaker is black, then sound is muted.