Learn Swedish: Du hör sirenen närma sig i mörkret. - You hear the siren approach in the dark.

This sentence uses a verb of perception: hör (hear). When you hear or see something doing an action, Swedish uses the object (sirenen) followed directly by the bare infinitive of the action verb. So we use just närma, not att närma. This is exactly like English, where we say "hear the siren approach", not "hear the siren to approach".

Why is the word sig here? Doesn't the sentence start with Du?

The verb meaning "to approach" is reflexive in Swedish: att närma sig. A reflexive pronoun always matches the "actor" of that specific verb. Even though Du (you) is the main subject of the sentence, it is sirenen (an "it") that is doing the approaching. The correct reflexive pronoun for "it" is sig.

Mörkret looks a bit unusual. What is the base noun for "dark" here?

The base noun is ett mörker (a darkness). Because it is an ett-word that ends in -er, it drops the final e before adding the definite suffix -et. That is why it shrinks down to mörkret (the dark). Just like English, Swedish naturally uses the definite form for the phrase i mörkret (in the dark).

How would the sentence change if I started it with I mörkret for emphasis?