Ona mówi głośno przez telefon.

Yes. Polish often drops subject pronouns when the verb already makes the subject clear.

So:

• Ona mówi głośno przez telefon.
• Mówi głośno przez telefon.

Both can mean the same thing.

Using ona often adds one of these effects:

• emphasis: She is speaking loudly
• contrast: She is speaking loudly, not someone else
• clarity in context, if several people are being discussed

So in everyday Polish, leaving out ona is very common.

Mówi is the 3rd person singular present tense form of mówić.

That means it is used with:

• on = he
• ona = she
• ono = it

Examples:

• On mówi. = He speaks / He is speaking.
• Ona mówi. = She speaks / She is speaking.

Notice that in the present tense, Polish verbs usually do not change for gender in the 3rd person singular. So on mówi and ona mówi both use mówi.

Because głośno is an adverb, and here it describes how she speaks.

• głośny / głośna / głośne = loud, as an adjective
• głośno = loudly, as an adverb

In this sentence, we are describing the manner of speaking, so the adverb is needed:

• mówi głośno = speaks loudly

If you used an adjective, it would need to describe a noun, not the verb.

This is the normal Polish expression for talking by phone or over the phone.

Literally, przez often means through, so przez telefon is close to through/by means of the telephone.

In natural English we say:

• on the phone
• over the phone

But Polish usually says:

• przez telefon

So this is something to learn as a fixed expression.

After przez, Polish normally uses the accusative.

So in przez telefon, telefon is accusative.

The reason it looks unchanged is that telefon is an inanimate masculine noun, and for many such nouns, the accusative singular is the same as the nominative singular.

So:

• nominative: telefon
• accusative: telefon

Even though the form looks the same, the case is still accusative.

Yes. Polish word order is more flexible than English word order.

The neutral order here is:

• Ona mówi głośno przez telefon.

But you could also hear:

• Ona przez telefon mówi głośno.
• Głośno mówi przez telefon.
• Przez telefon mówi głośno.

These versions may sound more marked, depending on what the speaker wants to emphasize.

Very roughly:

• głośno near the front can emphasize loudly
• przez telefon near the front can emphasize that it is happening on the phone

So the sentence can be rearranged, but the most natural basic version is the original one.

It can mean either, depending on context.

Polish present tense often covers both:

• She speaks loudly on the phone
• She is speaking loudly on the phone

If you need to know which one is meant, context decides.

So:

• general habit: Ona mówi głośno przez telefon = She speaks loudly on the phone
• happening now: Ona mówi głośno przez telefon = She is speaking loudly on the phone

It depends on what you want to say.

• mówi = speaks, is speaking, says
• rozmawia = talks, is talking, has a conversation

So:

• Ona mówi głośno przez telefon. = She is speaking loudly on the phone.
• Ona rozmawia przez telefon. = She is talking on the phone.

If the point is simply that she is engaged in a phone conversation, rozmawia przez telefon is often very natural.

If the point is specifically that her speech is loud, mówi głośno przez telefon works well.

It can be used with other actions too. It means something like by phone or over the phone.

Examples:

• Rozmawiam przez telefon. = I am talking on the phone.
• Zamówiłem to przez telefon. = I ordered it by phone.
• Załatwiliśmy to przez telefon. = We handled it over the phone.

So przez telefon is a broader expression, not only for speaking.

A rough pronunciation is:

• głośno ≈ gwosh-no

A few helpful points:

• ł sounds like English w
• ś is a soft sh-like sound
• the stress is on the second-to-last syllable: GŁOś-no

So it sounds roughly like GWOHSH-no, with the stress on the first syllable because it is a two-syllable word.

A rough pronunciation is:

• mówi ≈ MOO-vee

Helpful points:

• ó is pronounced like u
• w in Polish sounds like English v
• stress falls on the second-to-last syllable, so here on mó-

So mówi sounds roughly like MOO-vee.

A rough pronunciation is:

• przez ≈ pshesh or pshehsh

Important details:

• rz here sounds like the Polish sound written ż
• prz at the beginning is a difficult consonant cluster for English speakers
• the final z is pronounced like sh here because of sound changes in normal pronunciation

You do not need a perfect English equivalent, but something close to pshesh is a useful starting point.

Yes, it is natural.

It sounds like a normal sentence someone might say when describing another person's behavior.

For example, it could be used in situations like:

• complaining that someone is too loud
• describing what someone is doing right now
• stating a habit

If the context already makes the subject clear, many speakers would more naturally say simply:

• Mówi głośno przez telefon.

But the full version with ona is still perfectly correct and natural.