Game awareness is more than using headphones. In fact, I play with music in the background, making it less likely for me to hear your footsteps. Headphones are much more important when there are fewer total players. Hearing footsteps in a 1v1 is vital, hearing footsteps in a 10v10 is not as important. The more important point is map control.
Whether you're trying to do the objective or just playing for frags, you want to maintain map control. The longer you can stay alive the more time you have to deliver damage. Even if you cannot get the frag, it's important to deliver damage so the enemy cannot run over the rest of your team so easily. It's very important that you not get aced by a surprise attack. The way you prevent surprise is "knowing" where the enemy is at.
You can have a reasonable stochastic model of enemy positions as a function of the spawn times and average enemy travel speed. Sometimes these probabilities are certainties. If you know the enemy spawn time and you've been reading the obituaries and there's only one spawn point for the enemy, then there is a moment where you know their exact position. As time goes forward their position becomes less certain, assuming you are not directly observing them. You also know the limit of their maximum distance from the spawn, their wavefront, since you know their travel speed. For example, "Lotto" (lottery) refers to putting splash damage (can also refer to pre-firing) on the map, hoping to hit something. They don't know for certain that you will be there, only that it's probable. The "pro wait" guards against this: avoid being at the wavefront (maximum travel from spawn) as that is more likely to catch lotto damage.
Every map is different, but in general ET maps are more closed or indoors (they're not like baserace), giving rise to particular "routes" or "vias" which are pathways from the spawn. If you're defending, you want to cover as many routes as possible without "leaking" enemies, which is letting them travel past you unchecked on their way to the objective. If you think of the map as a graph, the edges are the corridors and the nodes are the openings where more than one edge meets. Some nodes dominate the path from the attacker's spawn to the attacker's objective, meaning every pathway goes through the node. As the defense you want to position on these nodes, guaranteeing you don't leak enemies. However, most maps are designed to avoid having dominator nodes like this to give the attack other options. I'm going to make up the word "dominance degree" to describe this characteristic of a node: the probability that an enemy travels through this node. You want to hold nodes with a high degree of dominance, but it's worthless to hold it if enemies leak through other paths that can flank this node. This requires communication and coordination on defense. For example, if after the enemy respawn none of them come through your node, it's very likely that they took another path (although it's possible they are waiting). At this point you should either rotate to the other node to help that side of the defense or you can gain more of the map by pushing up to hold a high dominance node even closer to their spawn. Keep in mind that such a tight hold on the spawn only pays out if your other defense does not fail -- because if they do, you will have a slow response to the objective as you must cross the map (use a killout to mitigate this). When you have a tight hold on their spawn it very often coerces the enemy to take alternative routes. After stopping their push on your route, look to push into their other likely routes for a flank and then reset (killout). Look around at what fraction of your team is on your route to make a reasonable guess as to whether the other node is sufficiently guarded. Keep in mind that if the attack is coordinated they can create a numbers advantage for themselves when your team is split like this.
As defense you're largely just fighting for these nodes (yes, you're basically camping). When you control these nodes, the enemy cannot get behind you, forcing the attack to muscle through choke points. If they do get behind you, you have to retreat or "dig" them out. Ignoring them just invites a flank. If you're attacking, this is a valuable thing to do for your team to make the defense split their attention (or just take the flank kills if they ignore you). Doling out unchecked damage is always a plus for your team.
To recap why map control is important:
- You don't want to take unchecked damage (you do want to deliver unchecked damage)
- You want to land the first hit in a duel. The more likely you are to have your crosshair on the enemy when rounding a corner the more likely you'll win the fight.
- You don't want to be outnumbered. Assuming a relatively small skill gap, you're going to lose a 2v1 gun fight. You can be happy about going 1:1, so long as there's no enemy medic to mitigate your kill.
- You want to force your enemy through choke points. This is basically the same as the previous point about being outnumbered. Choke points force your enemy into a 1v1 (ideally many to one if your team is helping) despite they're in a pack. Sometimes they even deliver team damage trying to get at you, thwarting their own push. Don't be this person. Likewise, don't let your teammates fight solo against multiple enemies. The side with the higher damage throughput is likely to win -- increase your team's throughput by increasing its damage bandwidth.