Amount of turns passed affect hearing?
Does the amount of time (turns) passed in a mission affect the chance to hear the enemy?
For example I think if I am slowly moving my squad around the map I can hear the enemy more often which is a large advantage. I would imagine a designer would put that in the game so you don't get too bored looking for the enemy.
I think hearing enemies is, like many other things in XCOM, a random thing. The chances are higher when you have no enemies in sight and you are far away from them. However, it's possible to hear enemies that are out of line of sight even when you are in combat with other enemies. It's also possible to have two soldiers hear enemies in the same turn. I even had a single soldier hear the same enemy twice with no delay between!
One thing that does seem consistent, however, is that when you hear an enemy, it will resonate from the closest enemy to you. I have never heard enemies from across the map when there are closer ones nearby.
So, your strategy of playing slow is probably just increasing the chances of hearing enemies, as you're taking more turns, which is indeed a useful thing.