Learn Hardcore French: Le son de la mer est calme.

son (meaning sound) is masculine, so it takes the masculine singular definite article le.
If the noun were feminine, you would use la (for example, la mer, the sea).

So you say le son because son is masculine, and la mer because mer is feminine.

Why is it de la mer and not du mer?

The choice depends on the gender of the noun that follows:

du = de + le (used before a masculine singular noun)
de la is used before a feminine singular noun

Since mer (sea) is feminine, you must use de la:

de la mer = of the sea
If the noun were masculine, like le vent (the wind), you would say du vent (of the wind).

What does de express in le son de la mer?

Here de expresses a relationship similar to of or from in English. It can indicate:

origin/source: le son de la mer = the sound that comes from the sea
possession or a belonging relationship: literally the sound of the sea

So de la mer tells you where the sound comes from or what it is associated with.

Could I say le bruit de la mer instead of le son de la mer? What is the difference between son and bruit?

Both are possible, but they have different nuances:

son = sound in a neutral or sometimes more pleasant / general sense
- le son de la mer can suggest a calm, soothing, perhaps more poetic idea.
bruit = noise, often with a less pleasant or more chaotic connotation
- le bruit de la mer might suggest louder crashing waves, or simply focus more on the noise aspect.

In your sentence, le son de la mer est calme fits well because it emphasizes the calm, gentle quality of the sound.

Does the adjective calme need to agree with son? Why does it look the same as in the feminine?

Yes, calme agrees with son, which is masculine singular:

son = masculine singular → adjective must also be masculine singular.

However, the adjective calme has the same written form for masculine and feminine in the singular:

masculine singular: un son calme
feminine singular: une mer calme

So you do not see any spelling change, but grammatically it is still agreeing with le son (masculine singular).

If it were plural, you would add -s:

Les sons de la mer sont calmes.
Les vagues sont calmes.

If I change the noun to plural, how does the sentence change?

You need to pluralize the article, the noun, the verb, and the adjective:

Le son de la mer est calme.
→ singular: le son, est, calme

Plural version:

Les sons de la mer sont calmes.
- les sons (plural of le son)
- sont (plural of est)
- calmes (adjective in plural form, add -s)

Is calme describing le son or la mer?

Grammatically, calme is describing le son, because:

The structure is: Le son de la mer est calme.
- Subject: Le son de la mer (the whole group)
- Head noun: son
- Verb: est
- Adjective: calme

The adjective agrees with the head noun of the subject, which is son (masculine singular).
The meaning in English might feel like the sea is calm, but in French you are literally saying the sound is calm.

If you wanted to clearly say that the sea is calm, you would say:

La mer est calme.

Can I change the word order and say Le son est calme de la mer?

No, that word order is not natural French.

Le son de la mer est calme.
- de la mer is directly linked to son and must come right after son.

Putting de la mer at the end (Le son est calme de la mer) sounds wrong to a native speaker. It looks like you are saying The sound is calm of the sea, which is not how French structures this idea.

Can I drop de la mer and just say Le son est calme?

Yes, grammatically you can say:

Le son est calme. = The sound is calm.

But you lose information: you no longer say which sound.
Le son de la mer est calme specifies that the sound you are talking about is the sound of the sea. Without de la mer, the sentence becomes more general and could refer to any sound.

How do I make this sentence negative in French?

To make it negative, you wrap the verb être (est) with ne and pas:

Affirmative: Le son de la mer est calme.
Negative: Le son de la mer n’est pas calme.

Details:

ne comes before est
pas comes after est
In spoken French, ne is often dropped: Le son de la mer est pas calme, but in standard written French you should write n’est pas.

How can I turn this into a yes–no question in French?

Common options:

Est-ce que question (very common, neutral):
- Est-ce que le son de la mer est calme ?
  = Is the sound of the sea calm?
Inversion (a bit more formal/written):
- Le son de la mer est-il calme ?
Intonation only (mainly spoken; same word order, rising intonation):
- Le son de la mer est calme ?

All three are understood; for learners, Est-ce que le son de la mer est calme ? is often the easiest to use correctly.