Answer:
They are all shutting out happiness.
Explanation:
The down keepers sound like they are feeling bad for themselves, and keep on feeling bad, or something along those lines.
The sun-slappers sound like they are pushing away the sun, or the light in other words, they are slapping away fun or happiness
The self-soilers sound like they are getting themselves dirty, and doing that on purpose.
And the harmony-hushers quiet the happiness, or singing or other things that make people smile.