Using your specific example, the Alert Character would immediately notice the loss of sound when the attacker moved into range. There is always ambient noise about, even if we tune it out, and a sudden silence would be startling.
I think the best way to look at Alert is to include acts of coincidence. In your example, assuming Boots of Elvenkind (which don't provide noise), the Alert character simply happens to glance in that direction at the right time. The feat has it's purpose, which is to prevent surprise, and going out of your way to nerf it seems unreasonable.
Now, that's not to say that it stops everything. For example, if you want to initiate a combat when a warlock summons demons, you don't have to call for initiative until AFTER the demons appear. The character isn't surprised, but it doesn't give them a chance to jump in and stop it either. Some players refer to this as a "cut scene," based on CRPGs, because the rules of the game don't apply during them. I don't suggest using it often, but it's something to consider.