They are quite vocal now here in Chon Buri. As you know, they can be vocal year round and at night, but usually moreso when the hormones kick in and they are looking for mates during breeding season(s). Both males and females vocalize. They do seem to go quiet during certain times of the year, but not for the extended stretches of months like the Plaintive Cuckoo.
Think you are correct though...if others are hearing them, then you're just tuning them out. When birding, especially in new areas, I tend to almost completely tune out the most common species in order to focus on less common/unfamiliar calls and songs. Later when compiling the checklist, have to really think as to whether I heard any Koels, Coucals, Plaintive Cuckoos, etc.