Ill start from the top
1) Flak Gun use: None. Unless you count running up beside a ship and shotting it's deck cargo to sink it a proper use.
2) torpedo speed just makes it harder for them to dodge but increases chances of breaching, duds and premeture blowing up.
3)Radar- Air radar shows enemy planes well before you can, giving you time to dive. Surface radar lets you see ships well before you can see them. This is helpful when attacking convoys becasue you can plan it before your in visual range.
4) Sonar- by thisx I assume you mean hydrophones so they let you hear enemy ships underwater. other uses are you can check depth below keel.
5) Yes it will become gradually louder also the closer they are the slower you will have to move the hydrophones needle, then it will suddenly speed up again when it goes over you or past you.
Hope this helped and welcome aboard
