sudo nvram SystemAudioVolume=%80

nvramtool: coreboot table not found. coreboot does not appear to be
 installed on this system.

7c436110-ab2a-4bbb-a880-fe41995c9f82-SystemAudioVolume

efivar --print --name 7c436110-ab2a-4bbb-a880-fe41995c9f82-SystemAudioVolume

GUID: 7c436110-ab2a-4bbb-a880-fe41995c9f82
 Name: "SystemAudioVolume"
 Attributes:
     Non-Volatile
     Boot Service Access
     Runtime Service Access
 Value:
 00000000  69        |i        |