I hate to ask the most obvious questions, but I will.
1) did you put the ball back in?
2) did you add credits?

There are two volume controls, if I recall correctly. Make sure that they are both turned up. If is not that, I would replace the capacitors on the sound board. Be careful with the ribbon connector between the speech board and the sound board.
For the flippers, check the end of stroke switches. If that doesn't fix it, check the plunger and coil sleeve. Clean and/or replace as needed. Also check the switches on the flipper buttons. Make sure that the contacts are clean.
I prefer the snappy feel of fliptronics flippers, but you can certainly rebuild those flippers.

Also, when you say that the drop targets don't work, what do you mean? Do they not reset, drop or register contacts?

That number means you are in audits mode. Change the batteries, or preferably, remove the need for batteries. Install an nvram or some other substitute.

Just replace it. Too cheap to try and fix: https://www.marcospecialties.com/pinball-parts/01-6452

Replace the coil sleeve, too.

I would start with replacing the capacitors. It will only take an hour or so. Are you ok with soldering?

Looks like GPE is out, but here: https://ksarcade.net/capacitor-kit-for-williams-system-6-7-sound-d-8224.html

Good point. Go ahead and read through Vid's Sys6 guide and plan on doing all of that at some point.

Everyone feels that way at the beginning. Take a breath, then start reading Vid's guide to bullet proofing system 6 machines. Start with what isn't working(the sound card), and repair that. Then move on. You'll get it.