Even though the speaker can read his little horse's mind, the horse can't talk back.
So, the next best option is to shake his booty. And by shaking his booty, we mean that he shakes his harness a little. There are little bells attached to his harness, which give a nice little jingle (think Santa Claus's sleigh).
To ask if there is some mistake.
Again with the mind reading. Our speaker knows his horse is shaking his bells in order to "ask" his master if something is awry, is there's a problem.
It's kind of like the horse is saying, "Hey, is everything OK? We've been standing here staring at nothing for a little while, and I just wanted to make sure you didn't need me to keep on truckin'. I'm cool with the standing still thing, but I just wanted to make sure I wasn't misinterpreting you."
The only other sound's the sweep
Beyond the harness bells' shaking, the only other sound that the speaker can hear is the "sweep."
The word "sweep" makes us think of the sound brooms make when they sweep dust into a dustpan.
At this point, we realize that the speaker is taking inventory of all of the sounds around him. He's interested in sounds.
Of easy wind and downy flake.
The sweeping noise comes from the slight wind and the softly falling snow.
Have you ever listened to snow falling? It's very, very quiet. There's just a gentle whirr. Everything is very, very still.