Learn Hardcore Korean: so-eumi simhamyeon gwimagaereul kkiseyo.

You use -면 after a vowel: 가다 → 가면, 크다 → 크면
You use -으면 after a consonant: 먹다 → 먹으면, 많다 → 많으면
Here, 심하다 ends in a vowel (하), so it becomes 심하
- 면 → 심하면.

Does 심하면 mean if it’s noisy or if it’s too noisy?

심하다 means to be severe/serious/intense. With 소음이 심하다, it often implies the noise is bad/serious, which many learners naturally translate as too noisy.
In context, it functions like: If the noise is bad/strong, …

Why is it 소음이 심하면 and not 소음이 심하면요 or 소음이 심하면은?

소음이 심하면 is a clean, neutral conditional clause.
소음이 심하면요 adds a conversational, softer tone (often used in speech to sound less blunt).
소음이 심하면은 is generally not used; -은/는 after a verb/adjective clause like that is not the normal pattern here.

What role does 이 play in 소음이?

이/가 marks the subject. 소음이 means the noise (as the thing that is severe).
You could also see 소음은 심하면…, which would shift the nuance to a contrast/topic like As for noise, if it’s severe…, but the most straightforward is 소음이.

Why is 귀마개를 marked with 를?

를/을 marks the direct object. Here, the action is 끼다 (to put on/insert/wear), and 귀마개 (earplugs) is what you put on, so it takes 를: 귀마개를 끼세요.

Why use 끼다 for earplugs? Could I use 쓰다 instead?

In Korean, 끼다 is commonly used for things you fit onto or into a body part:

장갑을 끼다 (put on gloves)
반지를 끼다 (put on a ring)
귀마개를 끼다 (put in/put on earplugs)

쓰다 is used for things you wear on the head/face:

모자를 쓰다 (wear a hat)
안경을 쓰다 (wear glasses)
Earplugs are typically 끼다, not 쓰다.

What does -세요 add to 끼다?

-세요 makes a polite request/imperative: please do… / (you should) do…
So 끼세요 is the polite form of telling someone to wear/put in earplugs.

Is this sentence a command, advice, or instruction?

Grammatically, -세요 is an imperative/request form, but in real use it often functions as advice or instruction, depending on context (signs, guidelines, staff directions). It’s polite, not harsh.

Can -(으)면 also mean when rather than if?

Yes. -(으)면 can mean:

if (conditional, hypothetical): If it’s noisy, wear earplugs.
when (general/typical situation): When it’s noisy, wear earplugs.
Here it commonly reads like general guidance: Whenever it’s very noisy, use earplugs.

What’s the dictionary form of 끼세요, and how is it formed?

Dictionary form: 끼다.
Polite imperative formation: 1) Take the verb stem: 끼-
2) Add -세요 → 끼세요
Note that 끼다 is a ㄲ verb and behaves regularly here.

How would I make it more formal, like something on a notice?

A common formal style is -(으)시기 바랍니다 or -(으)십시오:

소음이 심하면 귀마개를 끼시기 바랍니다. (Please wear earplugs.)
소음이 심하면 귀마개를 끼십시오. (Wear earplugs.)
These sound more official than 끼세요.

Is the subject you missing in the second part? Why isn’t 당신은 included?

Korean often omits subjects when they’re obvious from context. In 귀마개를 끼세요, the implied subject is you (the listener).
Using 당신 is often avoided because it can sound confrontational or overly direct in many situations.