For future readers Flash had two different rom versions - I guess because the production run was so long.

To work out which you've got open the coin door, switch to auto/up position and press advance.

P1 display now shows the rom version

0486 1 or 0486 2 is yellow roms, system 4
1486 1 is green roms, system 6

the game rom must match the flipper rom

Background sound is setting 33 with yellow roms, 31 with green roms. In both cases 0 = off (default) and 1 = on.
Other settings changed between rom versions but it's sound that seems to get noticed, particularly because it defaults to off.

It's odd to me that since background sound was the big new feature, first seen on Flash, that the default was off.