yoru ni soto de henna oto ga suru to, sukosi kowai desu.

に marks a point in time here, so 夜に means at night.

A few useful notes:

• Japanese time expressions sometimes take に, and sometimes do not.
• With 夜, both 夜 and 夜に can be natural, depending on style and nuance.
• Using に makes the time setting feel a little more clearly marked.

So 夜に外で... is simply setting the scene as at night, outside...

で marks the place where an event happens.

In this sentence, the event is a strange sound happening / being heard, so 外で means outside as the location of that event.

Compare:

• 外で変な音がする = a strange sound happens/is heard outside
• 外に行く = go outside

So:

• で = place of action/event
• に = destination, target, or location of existence in other patterns

Because 変 is a な-adjective, not an い-adjective.

That means:

• before a noun: 変な音 = a strange sound
• at the end of a sentence: その音は変です = that sound is strange

So the な appears only when 変 directly modifies a noun.

音がする is a very common expression meaning:

• a sound is heard
• there is a sound
• something makes a sound

It does not literally work like English the sound does something. It is a fixed Japanese pattern.

Similar expressions are:

• 匂いがする = there is a smell / something smells
• 味がする = there is a taste

So 変な音がする means there is a strange sound or a strange sound is heard.

In the pattern 音がする, 音 is the thing being perceived, and が marks it.

This combination is very standard:

• 音がする
• 声がする
• 匂いがする

You can think of it as: a sound occurs/is present.

If you changed it to 音はする, that would sound contrastive, like:

• 音はするけど、何かわからない = There is a sound, but I don’t know what it is.

So in a neutral sentence like this, が is the natural choice.

Here, と is a conditional meaning when or if.

So:

• 変な音がすると = when/if there is a strange sound

In this sentence, it gives a general reaction:

• When there’s a strange sound outside at night, it’s a little scary.

This と is often used for:

• natural results
• automatic reactions
• general truths
• repeated situations

It is not the quoting particle here.

少し means a little or slightly, and it modifies 怖い.

So:

• 少し怖い = a little scary / a little frightening

Then です makes the sentence polite:

• 少し怖いです = it’s a little scary

A helpful grammar point:

• 怖い is an い-adjective
• polite form: 怖いです
• negative: 怖くないです or 怖くありません

It can work in both ways depending on context.

In this sentence, 少し怖いです can naturally mean:

• it’s a little scary
• I feel a little scared

Japanese often leaves the experiencer unstated, so both ideas are wrapped together.

That is very common. The sentence does not need to explicitly say I am.

Yes. Japanese often omits the subject when it is obvious or not important.

So this sentence does not explicitly say:

• I
• it
• that situation

But from context, the meaning is something like:

• When there’s a strange sound outside at night, it’s a little scary
• When I hear a strange sound outside at night, I get a little scared

This kind of omission is completely normal in Japanese.

Yes, 変な音が聞こえる is also natural, but the nuance is a little different.

• 変な音がする = there is a strange sound / a strange sound is coming from somewhere
• 変な音が聞こえる = I can hear a strange sound

So:

• する focuses more on the existence or occurrence of the sound
• 聞こえる focuses more on the listener hearing it

Both are common, but 音がする is a very standard expression in sentences like this.

This is a very natural Japanese order: time often comes before place.

So:

• 夜に = at night
• 外で = outside

Putting them together as 夜に外で sounds smooth and normal.

Japanese word order is somewhat flexible, but this order is easy and common for learners to follow:

time + place + rest of sentence