Sound aggro monsters refers to monsters that will aggro if you run by them even while they're not facing you. Since they're not facing you, it can't be sight, they must be 'hearing' you run by. To avoid it, you just need to switch to walk mode and walk by.

Note: "Walking" while on a mount will still aggro them.

Caution: Some sound aggro monsters may also be sight, so you may still want to avoid walking directly in front of them.

How do you know what monsters are which? Trial and error.