Originally posted by ling.speed:

As for swamp, sound travels far. If you make too much noise things will slowly gather around you. So you either kill it fast enough till its clear or find a calm initial location.

This is not the whole truth. You are right that sound travels far, but that applies to all biomes and all enemy types. They hear very well. There is one thing special about swamps and that is that draugr and skelletons start to move into your direction without knowing you are there. I have tested this in sneak mode and they accurately followed me despite not seeing and hearing me. In my opinion this is an unfair and cheap mechanic that completely invalidates sneaking. Graydwarfes do the same btw. It sucks.

So long story short: When you get to a certain distance to mobs in the swamp they will head to you inevitably and that is one factor why the swamps are difficult for newbies who don´t know this and wonder why they are being swarmed.