Dog barking tones?

My neighbors dog will constantly bark around 5-7 in the evening. But when he barks, it sounds like he's in pain. It's not a whimper, but it sounds almost like he's sick. I've heard him bark like this for at least 3 months now, but since we live in the suburbs I've never met these neighbors, we're separated by woods. I feel really bad for the dog. It sounds like a donkey trying to Hee-haw, but it's the dog barking. Any suggestions?