Learn French: Nous avons travaillé dur sans que le son devienne trop aigu. - We worked hard without the sound becoming too high-pitched.

In the expression travailler dur (to work hard), dur acts as an adverb modifying the verb rather than an adjective describing the subject. When adjectives are used as adverbs, they are always invariable, meaning they never add an -s or an -e to match gender or number.

Why is the verb devienne in the subjunctive?

The conjunction sans que (without) always triggers the subjunctive mood in French, because it introduces a condition or outcome that was avoided or hasn't happened.

Could I just say sans le son devenir instead of sans que le son devienne?

No, because the two clauses have different subjects (nous in the first half, and le son in the second). You must use sans que plus the subjunctive when introducing a new subject. You can only use sans plus the infinitive (like sans faire de bruit) when the subject is the same for both actions.

Does aigu always mean high-pitched?

Not always. Aigu generally means acute or sharp. It can describe a physical shape (un angle aigu), an intense pain (une douleur aiguë), or, as seen in this sentence, a sharp, high-pitched sound.