They work in the coil test or they work with the flipper buttons in test mode?

Go to switch test and see if your flipper buttons are registering

What is happening is that your mods are causing your 12v power supply to sag and no longer provide enough power to your flipper opto boards for the opto's to work correctly.

Sound problem might just be a coincidence. Check the connections at the sound board for the speakers and at each speaker. The speakers are daisy chained so a break anywhere along the line = no sound.

The BR would have a part number on its side that you can use to identify it. Are you saying that the flippers actually move while in test mode but not during game play ? If that's the case there's no reason to think that the bridge is damaged otherwise it simply wouldn't work. Did you measure the voltage in AC between pins 1 and 3 of J901 on the fliptronics board? And then the voltage , in DC , between ground (J904 pins 4 and 5 have ground) and each side of the 4 fuses F901, 2, 3 and 4 ? Are all these voltages present?

What works in test mode? The T12 Flipper coil tests? These all work? Then if you go into T2 switch levels test and press and hold both flipper buttons, the right most column should have 6 out of the 8 switches pressed in. The display will cycle all closed switches and should identify 4 of those 6 switches , leaving the bottom 2 switches not labeled on screen (they are upper left and upper right opto switches).

If the coils fire in T12 test, the problem probably isn't the on fliptronics board.

If the switches don't appear during the T2 test, There's a very good chance the issue is at on of the flipper left/right flipper opto switch boards , or the connection between those and the fliptronics board. A broken solder joint on pin 6 or 3 of the left flipper opto switch board (or the wires going to those pins) will disable both flipper buttons at the same time. A broken solder joint on pins 7 or 4 of the left flipper opto switch board will disable the right flipper but would keep the left flipper working. Both the 12v and ground come in one pin and goes back out another to the right flipper opto switch board. Reflowing all pins on both the flipper opto switch board is a good measure.

NOTE: the left flipper opto switch board or the right flipper opto switch boards are not the same as the A-13901-1 Opto switch board assembly (that contains op-amps , diodes and etc). They are the boards that monitor the flipper buttons directly and have very few components , an opto directly on the board and some resistors, and only 1 connector J1.

Also , didn't you say your flippers started working again a few days ago after removing the piggy back but only sound was missing? And now it's back to both? Did you verify that the wire loop is still closed from J505 into the speaker panel , the connection out from the speaker panel down to the sub in the cabinet and then coming back out back to J504 ? If anything along that line is disconnected you will lose all audio.

Let's start by fixing the flippers before we delve into the possible sound issues.

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The flippers wouldn't work in test mode if the bridge were bad.

Look elsewhere as others have pointed you towards. For instance, go into switch matrix edge test and press your flipper buttons - the optos are part of the switch test, too, so you can see if they are working vs. not.

When you say they work in test mode do you mean the automated test will fire them but the buttons do not, or the buttons work to fire in test mode as well?