Learn Russian: Лекция в аудитории. - The lecture is in the lecture hall.

In Russian, the present tense of the verb "to be" (am, is, are) is usually skipped. You just place the subject and the location together, effectively saying "Lecture in lecture hall."

Why does аудитории end with a double и instead of the usual е for the prepositional case?

The word follows the preposition в (in) to show location, so it needs the prepositional case. The dictionary form is аудитория. While most feminine nouns get an -е ending in this case, nouns that end in -ия are a special exception and change to -ии instead.

Does аудитория mean a huge theater like "auditorium" does in English?

It can, but in Russian universities, аудитория is the standard word for any classroom or lecture hall, even relatively small ones. It is also used to mean the "audience" (the people listening).

How should I pronounce the preposition в before аудитории?

Because the next word starts with a vowel, the в is pronounced clearly as a "v" sound and merges directly into the noun. Say it as if it were a single word: "vauditorii".