Section II (Lines 9-14) Summary

Get out the microscope, because we’re going through this poem line-by-line.

I love to hear her speak, yet well I know
That music hath a far more pleasing sound;

Now, after all of that criticism, the speaker starts to get a little bit nicer.
He admits that he really does "love to hear her speak." Seems like she was due for a compliment, doesn't it?
The speaker can't just let it go at that, though, and immediately he starts to back up a little.
Basically, that "yet" in the middle of line 9 gets us ready for a negative comparison. It's like saying, "You're really great, but…"
Then, in line 10, we get the negative half of that thought: he thinks that music is "more pleasing" than the sound of her voice.
Well, maybe that's not so bad after all. If your boyfriend or girlfriend said, "I like music more than the sound of your voice," you might not exactly be thrilled, but it sure beats having him or her tell you that your breath stinks.
Maybe the speaker is softening up a little bit.

I grant I never saw a goddess go;
My mistress when she walks treads on the ground.

Here's another thought that is split over two lines. In line 11, the speaker essentially tells us that he's willing to admit that he's never seen a goddess move. (See why Shakespeare's the poet and not us? Listen to how smoothly those words flow together: grant…goddess…go. Nice, huh?)
Now, when the speaker finishes his thought on line 12, he's not actually being mean at all, just stating the facts. His mistress isn't a goddess, she doesn't fly or soar or float along. She just walks (treads) like a normal person, on the ground.
A pretentious poet might say: "My love walks like a goddess," but we would know that it isn't true. Has he ever seen a goddess? Maybe the best way to tell someone you love him or her in a poem is to be simple, honest and straightforward.

And yet, by heaven, I think my love as rare

As any she belied with false compare.

Now, at long last, we get to the sweet part, but it might take a little bit of translating.
Here are two lines in plain English: the speaker thinks that his lover is as wonderful ("rare") as any woman ("any she") who was ever misrepresented ("belied") by an exaggerated comparison ("false compare").
These last two lines are the payoff for the whole poem. They serve as the punch-line for the joke. They drive home the speaker's main point, that unlike other people who write sonnets, he doesn't need flowery terms or fancy comparisons. He can just tell his mistress, plainly and simply, that he loves her for who she is. Awww…