gassyou no ato de, minna ga ookina koe de 「yokatta」 to iimasita.

あと is a noun meaning after or the time after.

When あと follows another noun, you usually connect them with の:

• 合唱 = chorus / choral singing
• 合唱のあと = after the chorus

Then で turns the whole thing into a time expression meaning after the chorus or after the singing.

So:

• 合唱のあとで、… = After the chorus, ...

This is a very common pattern:

• 授業のあとで = after class
• 食事のあとで = after the meal

Yes. 合唱のあと、 would also be natural.

The difference is small:

• 合唱のあと、 = after the chorus
• 合唱のあとで、 = after the chorus / afterward

In many sentences, both are fine. あとで often sounds a little more like a complete time phrase.

Also note that あとで can sometimes mean later, depending on context, but here it clearly means after the chorus.

が marks みんな as the subject of the action: the people who said something.

So:

• みんなが言いました = everyone said it

Using が here is a neutral, straightforward way to describe what happened.

If you used は instead:

• みんなは言いました

that would sound more like as for everyone, they said..., which can add a topic or contrast nuance.

So が is very natural here because the sentence is simply reporting an event.

大きな声で means in a loud voice.

Breakdown:

• 大きな = big / loud
• 声 = voice
• 大きな声 = a loud voice
• 大きな声で = in a loud voice

The particle で here marks the manner of the action: how something was done.

So:

• 大きな声で言いました = said it in a loud voice

This で is not a location marker here. It tells you the way the speaking happened.

Similar examples:

• 小さい声で話す = speak in a quiet voice
• 日本語で話す = speak in Japanese

Both 大きな声 and 大きい声 are possible.

A useful way to think about it is:

• 大きい is the regular -い adjective
• 大きな is a special form used only directly before nouns

So:

• 大きい can appear at the end of a sentence: 声が大きい
• 大きな cannot: you would not say 声が大きな

Before a noun, both are often natural:

• 大きい声
• 大きな声

In this sentence, 大きな声 sounds very normal and natural.

Because it comes from いい / よい, whose past form is よかった.

• いい = good
• よかった = was good

But in real Japanese, よかった! is very often used as an exclamation, not just as a literal past-tense statement.

Depending on context, it can mean things like:

• That was great!
• It was good!
• I’m glad!
• That went well!

So even though it is grammatically past tense, it is a very natural reaction to something that has just happened.

と is the quotation particle.

It marks the content of what was said:

• 「よかった」と言いました = said “It was good!”

This pattern is extremely common:

• A と言う = say A
• A と思う = think A
• A と聞いた = heard that A

So here, と connects the quoted words よかった to the verb 言いました.

「 」 are Japanese quotation marks. They work like English quotation marks.

So:

• 「よかった」 = “That was great” / “It was good”

They show that these are the exact words people said.

Japanese can sometimes omit quotation marks if the sentence is still clear, but using 「 」 makes the direct quote very obvious.

言いました is the polite past form of 言う.

• 言う = to say
• 言った = said (plain form)
• 言いました = said (polite form)

The whole sentence is written in polite style, so the final verb is polite too.

If the sentence were written in plain style, it would be:

• 合唱のあとで、みんなが大きな声で「よかった」と言った。

Both are correct; the difference is politeness/style.

The sentence follows a very common Japanese order:

• 合唱のあとで = time expression
• みんなが = subject
• 大きな声で = manner
• 「よかった」と = quotation
• 言いました = verb

So Japanese builds the sentence piece by piece and puts the main verb at the end.

A natural breakdown is:

• After the chorus,
• everyone
• in a loud voice
• “That was great!”
• said.

That final-verb structure is one of the most important patterns in Japanese.