Mis amigas murmuraban en la última fila, pero se callaron enseguida.

Because amigas is the feminine plural form of amiga.

• mi amiga = my female friend
• mis amigas = my female friends

If the group were all male, or mixed male and female, Spanish would normally use amigos.

Also notice that mis does not change for gender here:

• mis amigos
• mis amigas

It only changes for number:

• mi = singular
• mis = plural

This is one of the most important things to notice in the sentence: it contrasts the imperfect and the preterite.

• murmuraban = imperfect
• se callaron = preterite

The imperfect (murmuraban) is used for an ongoing background action in the past:

• they were murmuring
• it was something in progress

The preterite (se callaron) is used for a completed action or a change:

• they fell silent
• they stopped talking

So the sentence paints this picture:

• first, there is an ongoing situation: Mis amigas murmuraban...
• then, something happens and that situation ends: ...pero se callaron enseguida

A natural way to think of it is:

• They were whispering/murmuring, but then they quickly went quiet.

Here murmuraban most naturally means something like:

• were whispering
• were murmuring
• were speaking quietly

The verb is murmurar.

Depending on context, murmurar can also mean:

• to mutter
• to murmur
• sometimes even to gossip or speak under one’s breath

In this sentence, because they are in la última fila and then se callaron, it strongly suggests quiet talking or whispering.

Because callarse and callar are not exactly the same.

• callar can be transitive: to silence someone/something
• callarse means to be quiet, to fall silent, or to shut up

So:

• callaron could suggest they silenced someone/something
• se callaron means they themselves became quiet

In this sentence, the reflexive se shows that the action affects the subject itself:

• Mis amigas se callaron = My friends fell silent / went quiet

This is very common in Spanish:

• Se sentó = he/she sat down
• Se durmió = he/she fell asleep
• Se callaron = they fell silent

Because en is used for location here.

• en la última fila = in the last row
• in the back row

The sentence is telling us where they were murmuring.

If you used a, it would usually suggest movement toward a place, not being located there:

• fueron a la última fila = they went to the last row

But here they are already there, so en is the natural choice.

La última fila means the last row or the back row.

• última = last
• fila = row

You often hear fila in places like:

• classrooms
• cinemas
• theatres
• churches
• any place with rows of seats

So en la última fila gives a very clear image: they were sitting or standing in the back row.

Because pero is the normal word for but when you are simply contrasting two ideas.

• Murmuraban..., pero se callaron...
• They were murmuring, but they fell silent...

Sino is used in a different kind of contrast, usually after a negation:

• No estaban hablando alto, sino murmurando.
• They weren’t speaking loudly, but rather murmuring.

So here there is no negation before the contrast, which is why pero is correct.

Enseguida means:

• right away
• immediately
• very soon

In this sentence:

• se callaron enseguida = they fell silent right away

It is a very common adverb in Spanish.

A few similar words are:

• inmediatamente = immediately
• rápidamente = quickly
• al momento = at once

But enseguida is especially common in everyday Spanish and sounds very natural.

Yes, you could say Mis amigas estaban murmurando, and it would also mean My friends were whispering/murmuring.

But there is a small difference in feel:

• murmuraban = simple imperfect; often used to describe background action in a smooth, narrative way
• estaban murmurando = emphasizes the action as being in progress at that moment

In many contexts both are possible, but murmuraban often sounds a bit more natural in storytelling.

So:

• Mis amigas murmuraban... = more neutral narrative background
• Mis amigas estaban murmurando... = highlights the ongoing activity a bit more

Because Spanish usually does not need subject pronouns when the verb form already makes the subject clear.

• murmuraban already tells you the subject is they
• se callaron also tells you the subject is they

And the noun phrase Mis amigas is already there, so adding ellas would be unnecessary in most contexts.

Spanish often leaves out subject pronouns unless there is a reason to include them, such as:

• emphasis
• contrast
• clarification

For example:

• Mis amigas murmuraban... = normal
• Ellas murmuraban... = more emphatic, like they were murmuring

No, the word order is very natural and standard.

The basic structure is:

• Mis amigas = subject
• murmuraban = verb
• en la última fila = place
• pero = contrast
• se callaron = second verb
• enseguida = adverb

So the sentence flows as:

• subject + verb + place, but
  • verb + adverb

Spanish word order is often flexible, but this version sounds straightforward and idiomatic.

For example, you could move enseguida:

• Mis amigas murmuraban en la última fila, pero enseguida se callaron.

That is also correct, but the original sentence is perfectly natural.

A natural Spain pronunciation would be roughly:

• Mis amigas murmuraban en la última fila, pero se callaron enseguida.

A few pronunciation points:

• amigas: the g before a is a normal hard g
• murmuraban: the r sounds are tapped, not like English r
• última: stress on úl
• fila: stress on fi
• callaron: in most of Spain, ll is pronounced like a y sound
• enseguida: stress on gui

In much of Spain, c before a, as in callaron, is a normal k sound, and ll is usually like the y in yes.