Learn Italian: Volete sapere la verità? - Do you want to know the truth?

Because it comes right after volete (you all want). In Italian, when you use a verb that expresses a desire, ability, or necessity, the action verb that follows it stays in the infinitive. "You want to know" is simply volete + sapere.

Why do we use sapere here instead of conoscere for "to know"?

Sapere is used when you know a fact, a piece of information, or a story, like la verità (the truth). You use conoscere when you are simply familiar with a person, a place, or a thing.

Who is being asked this question?

The verb volete is the voi (you all) form of volere (to want). This means the speaker is talking to two or more people. If they were asking just one person, they would say Vuoi sapere la verità?

Why does the word verità have an accent mark at the end?

In Italian, an accent mark on the final letter tells you to put the stress of your voice on that last syllable: ve-ri-TÀ. Without it, you would naturally stress the second-to-last syllable. Also, Italian words ending in an accented vowel do not change their spelling in the plural.