A policial disse que, se a sirene tocar de novo, ninguém deve atravessar a faixa de pedestres.

A policial means the police officer and specifically refers to a female officer here because of the article a.

• o policial = the male police officer
• a policial = the female police officer
• a polícia = the police force / police institution

So in this sentence, a policial is a person, not the police as an organization.

After the verb dizer (to say), Portuguese normally introduces reported speech or a reported statement with que.

• Ela disse que... = She said that...
• Ele falou que... = He said that...

So A policial disse que... simply means The police officer said that...

There is nothing unusual here: que is the normal connector for that in this kind of sentence.

Because Portuguese usually uses the future subjunctive after se when talking about a possible future condition.

So:

• se a sirene tocar = if the siren sounds
• se ele chegar = if he arrives
• se chover = if it rains

In English, we use the present after if: if the siren sounds.
In Portuguese, the standard form here is the future subjunctive.

For many verbs, the future subjunctive looks the same as the infinitive, which is why tocar may look familiar.

Yes. Tocar has several meanings depending on context. It can mean:

• to touch
• to play an instrument
• to ring / to sound when the subject is something like a bell, phone, alarm, or siren

So in this sentence:

• a sirene tocar = the siren sounds / goes off

This is a very normal use.

Here de novo means again.

So:

• tocar de novo = sound again

Yes, de novo and novamente often mean the same thing:

• A sirene tocou de novo.
• A sirene tocou novamente.

Both mean The siren sounded again.

In everyday Brazilian Portuguese, de novo is extremely common and natural.

Because ninguém means no one / nobody, and it is grammatically singular.

So the verb must also be singular:

• ninguém deve = correct
• ninguém devem = incorrect

This is the same logic as English nobody is, not nobody are.

Because ninguém already carries the negative idea: no one / nobody.

So:

• ninguém deve atravessar = no one should cross

You do not need não before the verb when ninguém comes before it.

Compare:

• Ninguém deve atravessar. = Nobody should cross.
• Não deve atravessar ninguém. = Nobody should cross.
  This version is possible, but the word order is different and less neutral in many contexts.

With ninguém before the verb, the simple standard pattern is ninguém + verb.

Deve is from dever, and with an infinitive it often expresses obligation, duty, or strong recommendation.

So ninguém deve atravessar can mean:

• no one should cross
• no one must cross
• no one is supposed to cross

The exact English choice depends on tone and context.
Because this is about a siren and a safety warning, it sounds fairly strong.

Because atravessar is normally a direct transitive verb in Portuguese. It takes a direct object without a preposition.

So:

• atravessar a rua = cross the street
• atravessar a ponte = cross the bridge
• atravessar a faixa de pedestres = cross the crosswalk

That is why you see a faixa de pedestres directly after the verb.

Faixa de pedestres is the usual Brazilian Portuguese term for a crosswalk or pedestrian crossing.

Literally, it is something like pedestrians’ stripe/marking, referring to the painted crossing on the street.

• faixa = stripe / band / lane / marked strip
• pedestres = pedestrians

So atravessar a faixa de pedestres means to cross using the pedestrian crossing area.

Because the expression is a fixed phrase: faixa de pedestres = pedestrian crossing / crosswalk.

The idea is that it is a crossing for pedestrians in general, not for just one pedestrian. Portuguese often uses this structure:

• sala de professores = teachers’ room
• escova de dentes = toothbrush
• faixa de pedestres = pedestrian crossing

So the plural pedestres is completely normal.

The conditional clause se a sirene tocar de novo is inserted in the middle of the larger reported statement, so commas help separate it.

Structure:

• A policial disse que ...
• se a sirene tocar de novo ...
• ninguém deve atravessar a faixa de pedestres

The commas make the sentence easier to read and show that the if-clause is an inserted condition.

Yes, but it would sound a bit more formal.

• se a sirene tocar de novo = if the siren sounds again
• caso a sirene toque de novo = in case the siren sounds again

Notice that with caso, Portuguese normally uses the present subjunctive:

• caso ... toque

With se for a future condition, Portuguese uses the future subjunctive:

• se ... tocar

Both are correct, but se is the more neutral everyday choice here.