Its not hard to learn but it does take time to become confortable with it. I recommend renting a car. Driving manual car for the first time with the added stress of driving in a different county is a recipe for disaster. By all means try it. Take up the offer of that lesson, but its unwise to have to rely on a manual car for transport when you've never driven one before.
But also make sure you actually need a car. The US is built with fully car dependent infrastructure, so americans are taught their whole lives to just assume they always need a car. That isn't the case everywhere, and might not be where you're going. Be sure to check.
Sound is complicated. Its both physics and perception.
The decibel scale is logarithmic. If you double the energy you increase the dB level by 3. Two people will be about 3 dB higher, four people would be 6, eight people would increase the level by 9 dB, sixteen would be 12 dB higher and so on.
Some people in this thread are talking about destructive interference. That really only happens in noise canceling headphones, or situations designed to cause it. In open air, its not really an issue that comes up. I've never become quieter by playing an instrument with someone else. It just doesn't work like that.
What lets the monster hear you is the peak volume, not the minimum or average, so it will be close to the 3 dB doubling rule.
There's also perception. Your brain does a ton of filtering of sound information. Your brain is constantly trying to pick out the important sounds and ignore the rest. That's why the quieter severs seem quieter when something louder happens. Also that's why some people use white noise machines to sleep.