Hljóðstyrkurinn er of hár, svo ég heyri þig ekki skýrt.

-inn is the definite article suffix (the “the” part) attached to the noun. So hljóðstyrkurinn literally means the volume (or the sound volume).
Without it, hljóðstyrkur would mean volume in a more general/indefinite sense.

Hljóðstyrkur is masculine. One clear clue here is the definite ending -inn, which is the masculine nominative singular definite ending for many masculine nouns (with some pattern variation across nouns).

Adjectives agree with the noun in gender, number, and case.
Here the noun is hljóðstyrkurinn = masculine, singular, nominative, so the adjective is hár (masc. sg. nom.).
If the subject were neuter, you’d typically see hátt; if feminine, há.

Of means too (excessive), not just very.
So of hár = too loud/high (i.e., beyond what’s acceptable).
For very, Icelandic more often uses mjög (e.g., mjög hár = very loud/high).

Yes, hár can mean tall/high, but Icelandic often uses “high” language for intensity levels: high volume → hár hljóðstyrkur. In English you can also say high volume, so it maps pretty naturally.

Because svo here functions like a conjunction meaning so / therefore, linking two clauses:

• Hljóðstyrkurinn er of hár,
• svo ég heyri þig ekki skýrt.
  A comma before clause-linking svo is very common.

No. Svo is flexible. It can mean things like:

• so/therefore (as here)
• then
• like this/that
• it can also appear in expressions like svo að (so that)
  Context decides the meaning.

Heyri is 1st person singular present tense of heyra (to hear):

• (ég) heyri = I hear
  It’s indicative present here (“I can’t hear you clearly”), not a special mood in this sentence.

Þú is the subject form (nominative) = “you” (as the doer).
Þig is the object form (accusative) = “you” (as the receiver).
In ég heyri þig (“I hear you”), you is the object, so Icelandic uses þig.

In Icelandic, ekki typically comes after the finite verb and often after objects in a neutral sentence:

• ég heyri þig ekki = I don’t hear you
  You can move things for emphasis, but this placement is very standard.

Because skýrt is being used as an adverb meaning clearly.
A very common way to form an adverb in Icelandic is to use the adjective’s neuter singular form:

• skýrt = clearly
  So it’s not agreeing with þig; it’s describing how the hearing happens.

Key points:

• Icelandic stress is usually on the first syllable: HLJÓÐ-styrk-ur-inn.
• hlj- is a cluster that can feel unusual; the h is part of the consonant onset.
• ð in hljóð is the voiced “th” sound (like in this) for many speakers/contexts.
  Also note ó is a long vowel.

• þ is like English th in think (voiceless). So þig starts with that sound.
• ð is like English th in this (voiced). It appears in hljóð.
  They’re separate letters in Icelandic and can change meaning if mixed up.