Hljóðbókin hjálpar mér að slaka á í strætó.

-in is the suffixed definite article for a feminine singular noun in the nominative:

• hljóðbók = an audiobook (indefinite)
• hljóðbókin = the audiobook (definite)

So the sentence is talking about a specific audiobook (or audiobooks in general, but phrased as the one in context).

Yes. hljóðbók is a compound:

• hljóð = sound, audio
• bók = book

Compounds are extremely common in Icelandic, and the first element often modifies the second (so literally sound-book → audiobook).

Because hjálpa typically takes the person being helped in the dative case, not the accusative.

• mér = dative of ég (me)
• mig = accusative of ég (me)

So Hljóðbókin hjálpar mér... is the normal Icelandic pattern: X helps me...

hjálpar is the 3rd person singular present tense of hjálpa:

• (ég) hjálpa = I help
• (þú) hjálpar = you help
• (hann/hún/það) hjálpar = he/she/it helps

Since hljóðbókin is 3rd person singular, you use hjálpar.

að is the common infinitive marker (often like English to):

• að slaka = to relax (in this construction)

It’s used after many verbs (including hjálpa) when the next verb is in the infinitive.

You generally should not drop it here. slaka á is the normal verb+particle/preposition combination meaning to relax / take it easy.

The á stays with the verb phrase, but when you add the infinitive marker að, you get:

• að slaka á (not að slaka)

This is similar in spirit to English “relax” vs “take it easy” (where easy is part of the expression).

It can also mean loosen or ease off, depending on context. For example:

• slaka á reipinu = loosen the rope
• slaka á (without an object) often = relax, unwind

In your sentence, it’s clearly the “relax” meaning.

í usually means in/into and commonly governs dative for location (in) and accusative for movement (into). Here it’s location: in the bus, so conceptually dative.

strætó is a very common colloquial word for bus and is often used in a way that doesn’t show much visible case ending in everyday speech/writing. You’ll also see more “fully declined” options like:

• í strætónum = in the bus (with the definite form)

Yes, strætó is informal/colloquial and extremely common in Reykjavík/Icelandic daily speech (also used as the name/brand for public buses). A more formal word is strætisvagn (bus).

The default structure is very similar to English:
Subject – Verb – Indirect object – Infinitive phrase – Prepositional phrase

• Hljóðbókin (subject)
• hjálpar (verb)
• mér (dative “me”)
• að slaka á (infinitive phrase)
• í strætó (where)

You can move elements for emphasis (Icelandic is flexible), but the verb still follows the usual verb-second tendencies in main clauses, so you can’t reorder completely freely without changing style or focus.