L’applauso del pubblico è fortissimo.

Because applauso is a masculine singular noun that starts with a vowel.
In Italian, the definite article il changes to l’ before singular nouns beginning with a vowel:

• il libro
• l’applauso

So l’applauso means the applause.

Del is a contraction of di + il:

• di = of
• il = the
• del = of the

So del pubblico literally means of the public / of the audience.

In this sentence, del pubblico tells you whose applause it is: the audience’s applause.

Here, pubblico is a noun, not an adjective. It means the public or the audience.

It is treated as a masculine singular noun:

• il pubblico
• del pubblico

So it does not change to match applauso. It stays pubblico because that is the noun being used.

The accent is very important because è and e are different words:

• è = is
• e = and

So in this sentence, è fortissimo means is very strong / is extremely loud, not and very strong.

Here fortissimo is an adjective describing applauso.

Base adjective:

• forte = strong, loud

Superlative form:

• fortissimo = very strong, extremely strong, very loud

Since applauso is masculine singular, the adjective is also masculine singular:

• applauso → masculine singular
• fortissimo → masculine singular

Both are possible, but they are slightly different in style.

• molto forte = very strong / very loud
• fortissimo = extremely strong / very loud, with a more compact and sometimes more emphatic feel

Italian often uses the -issimo ending to create a very common superlative:

• alto → altissimo
• bello → bellissimo
• forte → fortissimo

So fortissimo is a natural, idiomatic choice.

Because it agrees with the noun it describes: l’applauso.

Agreement in Italian means adjectives change form to match the noun in gender and number.

Here:

• applauso = masculine singular
• so the adjective must also be masculine singular: fortissimo

Compare:

• l’applauso è fortissimo
• la musica è fortissima
• gli applausi sono fortissimi

This is the normal Italian order:

subject + complements + verb + adjective

So:

• L’applauso del pubblico = the applause of the audience
• è = is
• fortissimo = extremely loud / very strong

Italian often keeps this straightforward order, especially in simple statements.

Yes, but it would mean something a little different in structure.

• L’applauso del pubblico è fortissimo focuses on the applause as a thing.
• Il pubblico applaude forte focuses on the audience doing the action of applauding.

So they are related in meaning, but not identical grammatically.

Yes. Applauso is singular here, just like English applause is usually singular.

So:

• l’applauso = the applause

Italian can also use the plural applausi when referring to separate rounds or instances of applause:

• gli applausi

But in this sentence, l’applauso is a singular mass/collective idea.

A helpful approximate pronunciation is:

lahp-PLAU-zoh del POOB-blee-koh eh for-TEES-see-moh

A few points:

• gli / gn are not in this sentence, so pronunciation is fairly straightforward.
• pubblico has stress on PUB.
• fortissimo has stress on TIS: for-TIS-si-mo.
• The è is pronounced like eh.

Also, l’applauso flows together smoothly because of the apostrophe.

Yes. In Italian, forte can describe both physical strength and intensity of sound.

So depending on context:

• vento forte = strong wind
• voce forte = loud voice
• applauso fortissimo = extremely loud / very strong applause

With applauso, English will often translate it more naturally as very loud or thunderous rather than just very strong.