Resonant of a sound means echoing or sustained. It means the sound continues a long time after Its source is finished. I suppose this meaning is meant for the music. It's possibly the same for the coastal house meaning the house causes the sound of the sea to echo inside it. However, resonant can also mean full of meaning and significance. This meaning might be meant - the house is full of cultural significance.

These are things that happened once, without anything happening at the same time. If you see 'while' then is likely continuous is needed. You mighyt have to think which is the most continuous action. Here they simply decided to visit the tower. Nothing happened while they were climbing the stairs. She turned around and then saw something.