Disable boot chime on late 2009 MacBook without macOS installed
The Start up chime resides in the NVRAM.

To disable it in OS X  run sudo nvram BootAudio=%00

I found this for Linux:

# efivar -l | grep SystemAudioVolume
 7c436110-ab2a-4bbb-a880-fe41995c9f82-SystemAudioVolume
 7c436110-ab2a-4bbb-a880-fe41995c9f82-SystemAudioVolumeDB

# efivar -n 7c436110-ab2a-4bbb-a880-fe41995c9f82-SystemAudioVolume -p
GUID: 7c436110-ab2a-4bbb-a880-fe41995c9f82
Name: "SystemAudioVolume"
Attributes:
    Non-Volatile
    Boot Service Access
    Runtime Service Access
Value:
00000000  00                                                |.               |

# efivar -n 7c436110-ab2a-4bbb-a880-fe41995c9f82-SystemAudioVolumeDB -p
GUID: 7c436110-ab2a-4bbb-a880-fe41995c9f82
Name: "SystemAudioVolumeDB"
Attributes:
    Non-Volatile
    Boot Service Access
    Runtime Service Access
Value:
00000000  00 
#


SOURCE