Arkadaşlar barda müzik dinledi.

Arkadaşlar is arkadaş + -lar.

• arkadaş = friend
• -lar / -ler = plural suffix, so friends

Turkish does not have words like a/an or the as regular articles. So arkadaşlar can mean:

• friends
• the friends

Context tells you which one is meant.

In Turkish, when the subject is already clearly plural, the verb often stays in the 3rd person singular form.

So both of these are possible:

• Arkadaşlar barda müzik dinledi.
• Arkadaşlar barda müzik dinlediler.

Both mean the same thing. The version without -ler on the verb is very common because the plural idea is already shown by arkadaşlar.

Barda means in the bar or at the bar.

It is made from:

• bar = bar
• -da / -de / -ta / -te = locative ending, meaning in / at / on

So:

• bar = bar
• barda = at the bar / in the bar

In this sentence, English would usually say at the bar, but Turkish uses the same locative ending for several English prepositions.

This is because of two things:

• vowel harmony
• consonant voicing

1. Vowel harmony

The last vowel in bar is a, which is a back vowel, so the suffix uses a rather than e.

So we get:

• -da, not -de

2. Consonant voicing

The word bar ends in r, which is a voiced consonant, so the suffix uses d, not t.

So:

• bar + da → barda

Here müzik is a bare object, which is very common in Turkish when the object is indefinite or general.

So:

• müzik dinledi = listened to music

This is like talking about music in a general sense, not a specific piece of music.

If you wanted to mean the music or that specific music, Turkish would usually use the accusative:

• müziği dinledi = listened to the music / listened to that music

Because Turkish uses the verb dinlemek differently from English.

In English:

• listen to music

In Turkish:

• müzik dinlemek
  literally more like music listen

So dinlemek takes a direct object. Turkish does not need a separate word meaning to here.

Dinledi is the simple past (often called the definite past in Turkish grammar).

It shows a completed action in the past:

• dinledi = listened

It is formed from:

• dinle- = listen
• -di = past tense marker

So dinledi means he/she/it listened, and in this sentence, because the subject is plural, it means they listened.

Yes:

• dinle- = verb stem, listen
• -di = past tense

So:

• dinle-di = listened

In 3rd person singular, Turkish does not need an extra personal ending here. That is why dinledi by itself can mean:

• he listened
• she listened
• it listened

And with a plural subject like arkadaşlar, it can also function as:

• they listened

Yes. It is a very natural Turkish word order.

The sentence is:

• Arkadaşlar = subject
• barda = place
• müzik = object
• dinledi = verb

Turkish often puts the verb at the end of the sentence. That is one of the most typical patterns in Turkish.

Other word orders are possible, but they usually change the focus or emphasis.

Yes. The Turkish locative ending -da/-de is broader than any single English preposition.

So barda can correspond to:

• at the bar
• in the bar

The best English choice depends on context. For a place like a bar, English often prefers at the bar, but in the bar is also possible depending on what you want to emphasize.

Because of vowel harmony.

The last vowel in arkadaş is a, which is a back vowel, so the plural suffix becomes:

• -lar

If the last vowel were a front vowel, it would be:

• -ler

So:

• arkadaş → arkadaşlar

Turkish would usually show that by adding the accusative ending to müzik:

• Arkadaşlar barda müziği dinledi.

Here:

• müzik = music
• müziği = the music / specific music

Notice that k changes to ğ before the vowel-initial ending. That is a common sound change in Turkish.

So the contrast is:

• müzik dinledi = listened to music
• müziği dinledi = listened to the music

Not by itself.

Arkadaşlar only means friends / the friends.

If Turkish wants to show possession, it normally adds possessive marking, for example:

• arkadaşlarım = my friends
• arkadaşları = his/her friends or their friends, depending on context

So in your sentence, plain arkadaşlar does not specifically mean my friends or their friends unless context adds that idea indirectly.