El audioprotesista calibra el audífono.

Because both nouns are singular, and in this sentence they are treated as masculine nouns.

• el audioprotesista = the specialist
• el audífono = the hearing aid

Spanish uses articles very regularly, often more than English does. So having el before both nouns is completely normal.

Also, audioprotesista is one of those profession words that can be masculine or feminine depending on the article:

• el audioprotesista = a male specialist
• la audioprotesista = a female specialist

Calibra is the third-person singular present tense form of calibrar.

The subject is el audioprotesista, which is he / the specialist, so the verb must match that subject:

• yo calibro = I calibrate
• tú calibras = you calibrate
• él/ella calibra = he/she calibrates

So El audioprotesista calibra... means the subject is doing the action in the present.

Good question. In Spanish, professions often appear without an article when they come after ser:

• Es audioprotesista. = He/She is a hearing-aid specialist.

But here, audioprotesista is not being used after ser. It is the subject of the sentence, so using the article is normal:

• El audioprotesista calibra el audífono.

So the difference is grammatical function, not just the word itself.

Yes. Audioprotesista is a profession noun ending in -ista, and many -ista nouns are the same for both genders.

The article tells you whether the person is male or female:

• el audioprotesista = the male specialist
• la audioprotesista = the female specialist

The noun itself does not usually change.

Because el audífono is a thing, not a person.

Spanish uses the personal a before a specific person (or sometimes a personified animal/pet) when that person is the direct object:

• Veo a María.
• Escucho al médico.

But with objects or things, you do not use that a:

• Calibra el audífono.
• Compra el aparato.

So el audífono appears directly, with no extra a.

Because the stress falls on dí: au-DÍ-fo-no.

This is an esdrújula word, meaning the stress is on the third-to-last syllable. In Spanish, all esdrújula words always have a written accent.

So:

• audífono ✓
• stress on dí

The accent mark tells you exactly where to stress the word.

A rough pronunciation guide is:

el ow-dyoh-proh-teh-SEES-tah kah-LEE-brah el ow-DEE-foh-noh

A few helpful notes:

• audioprotesista is stressed on sis: au-dio-pro-te-SIS-ta
• calibra is stressed on li: ca-LI-bra
• audífono is stressed on dí: au-DÍ-fo-no
• the r in calibra is a light Spanish tap, not a strong English r

A subject pronoun can often be omitted in Spanish, but a full noun subject like el audioprotesista is different.

You could say:

• Calibra el audífono.

But then the sentence just means He/She calibrates the hearing aid or Calibrate the hearing aid, depending on context and intonation. You lose the explicit noun the specialist.

So if you want to clearly say who does the action, keeping el audioprotesista is the normal choice.

It can mean either one, depending on context.

The Spanish simple present often covers both:

• a general/habitual action: calibrates
• an action happening now: is calibrating

So El audioprotesista calibra el audífono could mean:

• the specialist calibrates the hearing aid
• the specialist is calibrating the hearing aid

If you want to make the ongoing action very explicit, Spanish can also use:

• El audioprotesista está calibrando el audífono.

The plural would be:

Los audioprotesistas calibran los audífonos.

Changes:

• el → los
• audioprotesista → audioprotesistas
• calibra → calibran
• audífono → audífonos

So both the nouns and the verb change to match the plural subject.