Learn Turkish: Emin bir sesle konuştu. - He spoke in a confident voice.

In Turkish, when you use an adjective (like emin, meaning confident) and the word bir (a/an), the adjective almost always comes first. So "a confident voice" is built as "confident a voice" (emin bir ses).

What does the -le at the end of sesle do?

It means "with." It is a shortened, attached form of the word ile (with). Because ses (voice) has the vowel "e", ile follows vowel harmony and attaches as the suffix -le.

If sesle means "with a voice," why does the English translation say "in a confident voice"?

This is just a natural difference in how the two languages express things. While English speakers usually say they speak "in" a certain tone or voice, Turkish speakers say they speak "with" (-le) a certain voice.

Where is the word for "he" in this sentence?

It's built into the verb! The ending -tu on konuştu is the past tense ending for "he," "she," or "it." Turkish frequently drops the pronoun o (he/she/it) when it is understood. Without more context, this sentence could also perfectly mean "She spoke in a confident voice."