Otkad smo priključili novi zvučnik, glazba u učionici zvuči puno bolje.

Otkad means since or ever since when it introduces a clause.

In this sentence, it sets the starting point for a situation that continues into the present:

• first, we connected the new speaker
• from that time onward, the music sounds better

You may also see:

• od kada — a more separated form
• otkako — another common variant

All can mean something like since / ever since depending on context.

The part beginning with Otkad... is a subordinate clause. Croatian normally separates that kind of clause with a comma, especially when it comes before the main clause.

So the structure is:

• Otkad smo priključili novi zvučnik, = subordinate time clause
• glazba u učionici zvuči puno bolje. = main clause

If you reversed the order, you would usually still use a comma:

• Glazba u učionici zvuči puno bolje otkad smo priključili novi zvučnik.

Smo priključili is the perfect tense, the most common past tense in everyday Croatian.

It is formed with:

• a present-tense form of biti (to be)
• plus the l-participle

Here:

• smo = we are / auxiliary meaning we have
• priključili = past participle of priključiti

So smo priključili means we connected or we have connected, depending on context.

In this sentence, English naturally uses since we connected.

Croatian often leaves out subject pronouns when they are already clear from the verb form.

Here, smo already tells you the subject is we:

• (mi) smo priključili = we connected

So mi is unnecessary unless you want emphasis:

• Otkad smo priključili novi zvučnik... = neutral
• Otkad smo mi priključili novi zvučnik... = more emphatic, like since we were the ones who connected...

The past participle in Croatian agrees with the subject in number and, in the plural, also in gender.

Here the subject is an implied we:

• priključili = masculine plural or mixed-gender plural
• priključile = feminine plural

So if the speakers are:

• a mixed group, or gender is unspecified: smo priključili
• all women: smo priključile

This is very common in Croatian past-tense forms.

Priključiti means to connect, to hook up, or to plug in, depending on context.

With novi zvučnik, it most likely means physically connecting a new speaker to some device or system.

A useful contrast:

• priključiti = connect / plug in
• uključiti = switch on / turn on

So:

• priključili novi zvučnik = connected the new speaker
• uključili novi zvučnik would suggest turned the new speaker on

Because novi zvučnik is the direct object of priključili.

The verb priključiti takes an object in the accusative case.
However, for masculine inanimate nouns, the accusative singular looks the same as the nominative singular.

So:

• nominative: novi zvučnik
• accusative: novi zvučnik

That is why the form does not change.

If it were a masculine animate noun, you would see a clearer difference.

Because glazba is the subject of zvuči.

The thing that is doing the sounding is the music, so Croatian puts glazba in the nominative case.

Structure:

• glazba = subject
• zvuči = verb
• puno bolje = describes how it sounds

Zvuči is the 3rd person singular present tense of zvučati (to sound).

It agrees with the subject glazba, which is singular:

• glazba zvuči = the music sounds

Some other forms:

• zvučim = I sound
• zvučiš = you sound
• zvuči = he/she/it sounds
• zvučimo = we sound
• zvučite = you plural sound
• zvuče = they sound

Because this sentence talks about location, not movement.

With u, Croatian uses:

• locative for being in a place
• accusative for movement into a place

So:

• u učionici = in the classroom
• u učionicu = into the classroom

Here the music is sounding better in the classroom, so Croatian uses the locative:

• učionici

Because bolje is an adverb/comparative form, and it describes how the music sounds.

• zvuči bolje = sounds better

By contrast, bolja is an adjective:

• bolja glazba = better music

So compare:

• Glazba zvuči bolje. = The music sounds better.
• Ovo je bolja glazba. = This is better music.

After zvučati, Croatian normally uses dobro / bolje / loše / lošije, etc., to describe how something sounds.

Here puno means much.

So:

• puno bolje = much better

It intensifies the comparative bolje.

Croatian often uses puno this way in everyday speech:

• puno lakše = much easier
• puno brže = much faster
• puno gore = much worse

You could also hear mnogo bolje, which means basically the same thing.

In standard Croatian, glazba is the usual and preferred word for music.

Muzika is also understood, but:

• it is less standard in Croatian
• it may sound more colloquial, regional, or influenced by neighboring standards

So in a neutral standard Croatian sentence, glazba is exactly what you would expect.

Yes. Croatian word order is relatively flexible.

You could also say:

• Glazba u učionici zvuči puno bolje otkad smo priključili novi zvučnik.

That means the same thing.

The original version puts the time/background information first:

• Otkad smo priključili novi zvučnik...

This is very natural when the speaker wants to frame the main statement by saying ever since that happened...

It combines both ideas:

• priključili refers to a completed past action
• otkad links that past action to a situation that continues up to now

So the full idea is something like:

• ever since we connected the new speaker, the music has been sounding much better

That is why the perfect tense fits well here: the connection happened once in the past, and its effect is still relevant.