Η αδερφή μου προτιμάει να βάζει μουσική στο δικό της ηχείο, αλλά εγώ απολαμβάνω τη σιωπή.

In Greek, nouns almost always take a definite article, even when English wouldn’t use “the.”

• Η αδερφή μου = “my sister” (literally “the sister my”)
• η is the feminine singular definite article in the nominative case (“the”).

Without the article, αδερφή μου can sound incomplete or more like a form of address:

• Αδερφή μου, έλα εδώ. = “My sister, come here.”

As the subject of the sentence, αδερφή normally needs its article: Η αδερφή μου προτιμάει…

Both are understood and used:

• αδελφή is the more “standard” / formal spelling.
• αδερφή is a very common, more phonetic spelling in everyday writing and speech.

You will encounter αδερφή a lot in informal contexts (messages, social media, etc.), and αδελφή more in formal texts, schoolbooks, etc. Pronunciation is the same in modern Greek.

The verb can be conjugated in two parallel ways:

• προτιμά
• προτιμάει

Both forms are correct and mean exactly the same: “(he/she) prefers.”

In modern Greek:

• shorter form (προτιμά) is a bit more common in writing,
• longer form (προτιμάει) is very common in speech and in informal writing.

You can safely use either: Η αδερφή μου προτιμά μουσική or προτιμάει μουσική.

Greek often uses προτιμώ + να + verb to say “prefer to do something”:

• προτιμάει να βάζει μουσική = “she prefers to put on music / play music.”

Here:

• να βάζει (imperfective) shows a habitual action: she tends to put music on, she likes doing this regularly.

You could also say:

• προτιμάει μουσική – understandable, but sounds incomplete; you’d expect a comparison (e.g. “προτιμάει μουσική από τη σιωπή”).
• προτιμάει να παίζει μουσική – also good; παίζει (“plays”) focuses more on the music being played, while βάζει μουσική is very idiomatic for “putting music on” (turning on a speaker, starting a playlist, etc.).

The difference is aspect (imperfective vs. perfective):

• να βάζει μουσική (imperfective) = repeated / habitual / ongoing action
  → “she prefers putting music on (in general, as a habit).”
• να βάλει μουσική (perfective) = one specific, complete action
  → would sound more like “she prefers to (go and) put some music on (this time).”

In your sentence, we are talking about a general preference, so the imperfective να βάζει is the natural choice.

Breakdown:

• σε + το = στο = “in/on/at the”
• δικό της = “her own” (neuter singular form of δικός)
• ηχείο = “speaker”

So στο δικό της ηχείο = “on her own speaker.”

Difference:

• στο ηχείο της = “on her speaker” (neutral possession)
• στο δικό της ηχείο = “on her own speaker” (emphasizes that it is specifically hers, vs someone else’s, shared, mine, etc.)

Greek uses δικός/δική/δικό + μου/σου/της... when you want to stress “one’s own X.”

δικός / δική / δικό agrees with the thing owned, not with the owner.

Here the owned noun is:

• το ηχείο – neuter singular

So the possessive must also be neuter singular:

• το δικό της ηχείο
  δικό (neuter) agrees with ηχείο (neuter).

If the noun were feminine, you’d use the feminine form, e.g.:

• η δική της τσάντα = her own bag
• το δικό της ηχείο = her own speaker

στο is a contraction of two words:

• σε = “in / at / on / to” (very general preposition)
• το = “the” (neuter singular, accusative)

So:

• σε το ηχείο → στο ηχείο

Similarly:

• σε + τον = στον (e.g. στον φίλο μου)
• σε + την = στη(ν) (e.g. στη δουλειά)

The subject εγώ (“I”) is not strictly necessary:

• Απολαμβάνω τη σιωπή. = “I enjoy the silence.”

However, Greek uses the explicit pronoun εγώ to add emphasis or contrast. In this sentence we have a clear contrast:

• Η αδερφή μου … but εγώ …

So εγώ stresses “but I (on the other hand) enjoy the silence.” It mirrors the contrast “she vs I.”

• αλλά = “but” → expresses contrast.
• και = “and/also” → adds something similar.

The meaning here is:
“She likes putting music on her speaker, but I enjoy silence (instead).”

So αλλά is appropriate because your preference is the opposite of hers.
και εγώ απολαμβάνω τη σιωπή would mean “and I also enjoy the silence,” implying agreement rather than contrast.

Greek uses the definite article much more often than English.

• τη σιωπή (accusative, feminine singular with article) is the normal way to refer to “silence” as a general concept in this kind of sentence.

Literal:
απολαμβάνω τη σιωπή = “I enjoy the silence.”

You can say απολαμβάνω σιωπή without the article, but that sounds more bare/poetic or like “I enjoy (some) silence” as a mass, not the neutral everyday phrasing. The version with the article is the most natural in standard speech.

ηχείο is a neuter noun in Greek:

• το ηχείο – the speaker (nominative/accusative singular)
• του ηχείου – of the speaker (genitive)
• etc.

The article το shows that it is neuter.
η is the feminine article; it would be used with feminine nouns like η πόρτα (“the door”), η καρέκλα (“the chair”). Since ηχείο is neuter, it must take το.