There are a couple of things that may help with saddle placement. The first is knee over pedal (KOP). This means that when your pedal is at the three o'clock position, your knee should be roughly over the axle of the pedal. Then, as a starting point, you want your saddle to be high enough that you are not reaching for the pedals when in the down stroke with a slight bend of the knee. This should help with a start position but you'll need to fine tune it so you are comfortable. Another method is try an on-line fit calculator (like at competitive cyclist). This will give you more precise measurements to start out with but, again, you'll need to fine-tune the position.

As for the hearing on the trail, the only time it's helpful to me is to know when other riders are behind me. It's curtious to let them by when you've been riding without anyone behind you and they appear. It means they're faster. However, if someone is behind you and wants to pass, they will usually say "rider back" or some indication they are back there. When passing, they will, typically, call out the side they are passing on "on the left/right". But if the trails not that busy, you don't need to worry too much about that. I live in suburbia where the trail can get pretty busy. Not hearing these signals would be no different than a rider jamming music so it's probably not a big deal.