What should I do with my mac's volume?
I searched for the icon you've uploaded… 
If you see output devices in 'System preferences > Sound > Output', try selecting again the build-in internal speakers.


If 'System preferences > Sound > Output' shows no output devices, try a reset of the nvram.

Reference:


'volume stuck on mute, cant untick the mute box in sound settings (macbook air)' https://discussions.apple.com/thread/8593873   
'Reset NVRAM or PRAM on your Mac' https://support.apple.com/en-us/HT204063
'iMacの音量表示が斜線になりiMacから音が出なくなった時に試したこと' https://www.chawanmushi.net/entry/2018_0602-imac-sound-setting/