Volume icon in menu bar disappear randomly in Big Sur
Solution 1:
I know why this happens. Upon clicking the volume icon in the menu bar, if you misclick/misdrag anywhere but the allowable volume change slider it will interpret this as trying to rearrange the menu bar icon to another menu bar location.
My current "fix" is to use the open the control center menu and drag the volume back into the menu bar every time this happens.
EDIT: Seems to be fixed in MacOS 11.3
Solution 2:
I also have the same problem; I need to go to System Preferences -> Sound -> Show volume in menu bar every the other day!