Ako se opet budeš žalio na glazbu, podržat ću te, jer ni meni ne odgovara kad svira preglasno.

After ako referring to a future situation, Croatian normally uses future II, not future I.
Budeš žalio is part of that pattern: budeš + žalio.

So Ako se opet budeš žalio... means If you complain again....

English does something a bit similar in meaning, because you normally say If you complain again, I’ll support you, not If you will complain again.

In future II, Croatian uses the l-participle, and that participle agrees with the subject in gender and number.

So:

• to a man: budeš se žalio
• to a woman: budeš se žalila
• to more than one person: budete se žalili / žalile

So the form žalio shows that the person being addressed is masculine singular.

The verb is žaliti se, which means to complain.

Without se, žaliti usually means something else, such as to pity, to feel sorry for, or to regret, depending on context.

So:

• žaliti se na glazbu = to complain about the music

Se is a clitic, and Croatian clitics usually appear near the beginning of the clause. That is why it comes right after ako here.

Croatian word order is fairly flexible, but clitics like se, te, ga, ću tend to stay in these early positions.

After žaliti se, Croatian normally uses na + accusative for the thing you are complaining about.

So:

• žaliti se na glazbu = to complain about the music

Glazbu is the accusative singular of glazba.

This is simply the pattern that this verb requires.

This is future I.

Croatian can form future I with the infinitive plus a clitic form of htjeti:

• podržat ću
• vidjet ću
• napisat ću

When ću/ćeš/će... comes after the infinitive, the final -i of the infinitive is dropped in standard spelling.

So:

• podržat ću te = correct
• podržati ću te = not standard

You can also say Ja ću te podržati, where the auxiliary comes first and the full infinitive stays unchanged.

Because podržati takes a direct object, and the direct-object clitic for you is te.

Ti is usually the dative form, meaning to you.

Compare:

• podržat ću te = I will support you
• reći ću ti = I will tell you

Not in this sentence.

Odgovarati can mean several things in Croatian. Here it means to suit, to be acceptable, or to work for someone.

So ni meni ne odgovara means something like:

• that doesn’t work for me either
• I’m not okay with that either

So this is not the answer/reply meaning of odgovarati.

Because odgovarati takes the person affected in the dative case.

So:

• meni = to me

The usual pattern is:

• To mi odgovara = That suits me
• To mi ne odgovara = That doesn’t suit me

So meni is exactly the form Croatian expects with this verb.

Yes, and that is completely normal in Croatian.

Croatian regularly uses double negation:

• ni meni ne odgovara = it doesn’t suit me either
• nitko ne zna = nobody knows

So ni ... ne ... is standard Croatian, not a mistake.

The subject is understood from context. It could be the music, the stereo, the band, or whatever is producing the sound.

Croatian often leaves the subject unstated when it is already clear enough.

Also, svirati can mean to play in the sense that music is playing, not only that a person is playing an instrument.

Preglasno means too loudly.

It is made from:

• glasno = loudly
• prefix pre- = too, excessively

So:

• svira preglasno = it is playing too loudly

Here kad means when or whenever.

It is the shorter, very common form of kada. In most everyday contexts, kad and kada mean the same thing, though kada can sound a bit more formal.

So:

• kad svira preglasno = when it plays too loudly

Croatian uses commas to separate subordinate clauses quite regularly.

Here:

• Ako se opet budeš žalio na glazbu, ...
  The if-clause comes first, so it is followed by a comma.
• ..., jer ni meni ne odgovara ...
  Jer introduces a reason clause, so it is also separated by a comma.

So the commas are marking the sentence structure very clearly.