Он будет подпевать, если только не сорвёт голос.

Because подпевать is an imperfective verb.

In Russian:

• imperfective verbs form the future with быть + infinitive
• perfective verbs form the future with a single finite verb

So:

• подпевать → будет подпевать
• compare a perfective verb like подпеть → подпоёт

Here, будет подпевать suggests an ongoing or repeated action: he will be singing along / will sing along.

Подпевать means to sing along, to hum along, or to join in singing.

It comes from:

• петь = to sing
• prefix под- here gives the idea of singing along with something or someone

So Он будет подпевать means that he will join in vocally rather than perform as the main singer.

Depending on context, подпевать can mean:

• sing along to a song
• sing backup / in accompaniment
• quietly join someone else’s singing

Because сорвёт is the perfective future of сорвать.

Here the speaker is talking about a possible result:

• not an ongoing process of damaging the voice
• but the moment/result of straining/ruining/losing it

That is why Russian uses the perfective verb:

• сорвать голос = to strain or ruin one’s voice

So:

• будет подпевать = ongoing action
• сорвёт голос = single completed result that may happen

This contrast is very typical in Russian.

Если только не is a set expression meaning unless.

So the sentence means:

• He will sing along, unless he strains his voice.

Literally, it looks like:

• if only not

But in actual usage, the whole phrase functions like unless.

This is an important expression to recognize as a unit.

Because если только не is an idiomatic conjunction.

The не is part of the expression and does not simply negate the verb in the usual way. Together, если только не means:

• unless
• except if

So you should understand the whole chunk together:

• если только не сорвёт голос = unless he strains his voice

A learner often tries to read it word by word, but here that can be misleading.

It is a common expression meaning:

• to strain one’s voice
• to ruin one’s voice
• to lose one’s voice from overuse

It is partly idiomatic. Literally, сорвать often means something like to tear off, to pull off, or to rip off, but in the phrase сорвать голос it means to damage the voice by shouting or singing too much.

So in this sentence, it means he might sing so much that his voice gives out.

Голос is in the accusative singular because it is the direct object of сорвёт.

Base form:

• голос = voice

In this sentence:

• сорвёт голос = strain the voice

Because голос is a masculine inanimate noun, its accusative form looks the same as the nominative:

• nominative: голос
• accusative: голос

So the form does not change, even though the case does.

Russian often can omit subject pronouns when the meaning is clear from context, but including он is completely normal.

So:

• Он будет подпевать... = neutral, explicit he
• Будет подпевать... = possible if context already makes it obvious who is meant

Including он may help:

• make the subject clear
• contrast him with someone else
• sound more complete as a standalone sentence

This is a very natural, neutral word order.

Structure:

• Он будет подпевать = main clause
• если только не сорвёт голос = condition / exception clause

Russian word order is flexible, but this version is straightforward:

• first say what he will do
• then add the condition that could prevent it

You could move things around for emphasis, but this order is the most standard in ordinary speech.

Yes. The letter ё shows both pronunciation and stress:

• сорвёт is pronounced with stress on the last syllable

In many printed texts, Russian often writes е instead of ё, so you may sometimes see:

• сорвет

But it still means сорвёт.

For learners, it is helpful that ё is written here, because it makes the pronunciation clearer.

Yes, that is a very natural translation.

The Russian aspects support that idea well:

• будет подпевать = ongoing/repeated action
• сорвёт голос = one resulting event

So good English renderings include:

• He will sing along unless he strains his voice.
• He’ll keep singing along unless he loses his voice.
• He’ll be singing along, unless he ruins his voice.

The exact English wording depends on context, but the Russian grammar points to that general meaning.