Hold down +
while powering on the system.
Using controller 2 throughout, hold down +
as the title screen appears (the screen with the gun), then press any button to access and use the sound test.
This also activates a stage select (press /
on the round screen), stage skip (press
on controller 2 in-game) and extra lives (press
on controller 2 to increase lives and
on controller 2 to decrease lives).
See the main page for Line of Fire