This is troubleshooting. And the essence of the process is to isolate the problem. You plug a cord into it, touch the tip of the cord, and you hear hum/noise? That tells me the amp is working. It may not work well, but it should make noise with a guitar connected. Even though this guitar works, try a different one, or a bass, or a microphone, or a tape deck, play ANYTHING through it. We want to be sure it doesn't pass signal before we go further.
If plate voltages are lower than the B+ they came from, then the tubes are conducting current. But the other half of the reading is the cathodes (pins 3 and 8) if they are zero, then the tube is not working. If they are a volt or two, that is normal. The phase inverter tube will be higher than a volt or two.
Keep your eye on the prize - the issue is no sound, not bad sound or weak sound. So the problem is not going to be an off value part or a weak tube. Leaky caps still make sound. 100k resistors that are now 200k will still make sound. "No sound" generally means some sort of gross failure. So we look for basic stuff first. And the first thing is to make sure whether the problem is really inside the amp or not.