A sound bite is a short excerpt of speech that can be used to encompass the tone or thought of the entire piece. Yes, typically it is an audio clip, but bloggers can apply it to their writing. We have all seen the back cover blurbs that tout the laurels of the book. They are usually short - "Author has outdone himself", "Amazing...a work of art", etc. Much like you think about keywords when writing your post, pay attention to make there is a phrase or sentence that can be used to sum up for review. Actually, it can play a part in your SEO.
You may notice in my blog titles I have a short phrase that usually sums up my thoughts on a book. I make sure to repeat it in my first paragraph and then usually somewhere towards the end. Search engines love that kind of thing. And publishers/publicists love the ready made quote they can share on their social media and press material.
I will caution you: Do NOT write your review with the thought of being quoted. It will sound forced. Remember to be honest. If you didn't think it was "amazing", "Pulitzer Prize winning writing", or any of those overused hyperbole type phrases, do NOT use them. Sound bites should sound effortless and natural to your writing style.
A great way to practice crafting your sound bites and create content for your social media is to write your review in 140 characters or less. This is also one of those SEO/ promoting tips the pros tout. You should have 5 alternative "titles" for your post that you use when promoting on twitter and facebook. My friend Coral of Alchemy of Scrawl is participating in a 140 character book review challenge. She got the idea from Indie film maker Kevin Smith. She wrote a blog post over at Book Bloggers' Collaborative with other useful info from Kevin Smith. Here is another post about writing 140 character book reviews.
If you perfect the sound bite you may find yourself being quoted in press releases or even on the back cover of your favorite author's next book.