There are many different ways to get this info, but not all of them work equally well on all platforms.
Try this: (Windows&Linux)
unique values of (it as string as trimmed string) of values "bios_version" of structures "bios_information" of smbioses
OR: (Windows Only)
unique values of string values of selects "SMBIOSBIOSVersion FROM Win32_BIOS" of wmis
Genenally SMBIOS will be faster than WMI.