My son spotted that a column (albeit a zigzag) of LEDs wasn't lighting up on these funky pair of shades that react to sound.
As you might have seen on Christmas lights, if one LED goes out, the entire line goes out - which was the case here.
The test process to isolate the right LED was to put the multimeter into diode mode (or continuity mode also works) and put the positive lead to the positive leg and negative to the negative leg - which causes it to light up.
Working my way through the row of LEDs I found that one wasn't lighting up, I resoldered it (in the hope that it was just a bd connection rather than a duff LED) and hey-ho-what-do-you-know, it worked again 😎
