1) Sound - does your monitor contain speakers, or are you using separate ones ? Make sure they are in the right sockets on the back of the PC, it is very easy to have them shifted out by one place. If your sound card sockets are colour coded, the speaker is the green one and the microphone is red.
Do you have only one set of speaker sockets on the PC or two sets ? (Sometimes PCs with a sound card fitted also have sound sockets on the motherboard - which would be disabled. Make sure you are plugged into the right ones.)
2) This is unlikely to be anything to do with the monitor as they are completely unrelated, but it's possible you disturbed something when fiddling with the wires. Check your network cable from the router to the PC before doing anything else. I assume the lights on your router are normal ?
If that fails, go back to first principles and unplug everything, then plug back in one item at a time starting with power cable, monitor cable, keyboard and mouse .... try starting the machine.
Then add in the speakers
Then add in the network cable to the router and any other USB cables etc.
Of course there could by coincidence be a problem with your router or with your internet provider, so if nothing works try again after 24 hours.
Most important is take it slowly and don't panic, haste is your biggest enemy.