We change our behaviour when we start searching by speaking, so our voice searches are different. Our searches become more conversational and more "question" like.
This means they become longer. Normally, if we search by typing text into a search field we use 2-4 keywords. When we use voice searches, this can more than double to 6-10 words.
Our questions start with words such as who, how, when, where, why and what. Understanding the choice of word can give you insight into the user intent and what stage they are at in the buying process.
"What is better, a sony led tv or a samsung led tv?" shows someone in the research phase. Whereas, "Where can I buy a sony bravia 49" led tv?" indicates someone ready to buy.