I haven't been able to test much (most people just run forward and catch the aggro when I try), but I think enemies in dungeons just have both sight and sound aggro. If you're walking behind some enemies it doesn't seem to trigger, but the moment you speed up or they turn around you're caught.
I made a bug report for the overworld (wasn't accepted though), where only non-aggro enemies get the sound aggro icon (that is worthless on them because they don't aggro), while the sound aggro enemies just get the sight aggro icon. It's funny walking into the middle of a group of, for example, sprites or efts to pick off the enemy I want, and then some random person just runs right in, catches aggro from everything, and dies right at my feet (usually in a FATE).
