I don't have my bird books with me, I'm not in Thailand right now. But I believe I've read that the Scarlet-backed flowerpecker can be found throughout the whole country and is considered a common bird. After my first sighting I was also surprised to read that but I'm not a true birder and usually don't take the time to ID little birds hopping around in the trees. I suppose that when you pay more attention to birds that you will see it more frequently. Besides in the national park, I've also seen it in my garden in Pak Chong.
The Laughing thrush is quite common inside the park, though not the most common laughing thrush species. I've also seen it a lot in HKK which is at lower elevation and in different type of forest than in Khao Yai so I guess they are wide-spread and quite common.
The thrush was back this morning, along with a kingfisher. We are a few hundred meters from KY, there seems to be more bird life around, including several that I haven't seen before. I suspect the dry conditions are attracting them? Our house is very open, the second floor is great for birding.