Learn German: Sprecht bitte laut! - Please speak loudly!

This is an informal plural command, meaning it is addressed to a group of two or more people you know casually, like friends or children. It is based on the ihr (plural "you") form of the verb.

Why isn't there a word for "you" in this sentence?

For informal commands, both the singular du and plural ihr, German drops the pronoun entirely. If this were a formal command, you would keep the pronoun and say Sprechen Sie bitte laut! instead.

I learned that sprechen changes its vowel to an "i" (like du sprichst). Why is it sprecht with an "e" here?

The vowel change from "e" to "i" only happens in the singular forms like du and er/sie/es. The plural ihr form does not change its vowel (ihr sprecht). Because this is an informal plural command based on the ihr form, it keeps the "e".

Why does German use laut (loud) instead of a special word for "loudly"?

In German, adjectives (words that describe things) and adverbs (words that describe actions) use the exact same base form. You do not need to add a special ending like "-ly" in English. So, laut means both "loud" and "loudly" depending on how it is used in the sentence.