What does the “test result indicator” indicate on a cockpit voice recorder?
I could not find the exact model you showed, but the A330 and A340 FCOM on SmartCockpit shows a similar one. Unfortunately the description of the test is rather short:



The Overhead Pushbutton explanation on SmartCockpit is equally short:



My best guess would be that it shows the recorded volume from low on the left to high on the right and it is considered OK (test passed) if it reaches into the green LEDs, therefore making sure the CVR can pick up voices clearly enough. I hope someone working with these CVRs can provide a more accurate answer!