Learn Spanish (Latin America): Nos prohíbe tocar la trompeta en la sinfonía. - He forbids us to play the trumpet in the symphony.

Yes! With verbs of influence like prohibir, permitir, and mandar, you have two options. You can use an indirect object pronoun plus the infinitive (like in this sentence: nos prohíbe tocar). Alternatively, you can use que plus the subjunctive: Nos prohíbe que toquemos la trompeta. Both are perfectly natural and mean exactly the same thing.

Why is nos placed before prohíbe? Could it be attached to the end of the infinitive, like prohíbe tocarnos?

No, it must go before prohíbe. Here, nos is the indirect object of the main verb—it tells us who is receiving the prohibition. It doesn't belong to the action of playing (tocar), so you cannot attach it to the infinitive. Saying prohíbe tocarnos would bizarrely mean he forbids someone to play (or touch) us.

Since us (nos) is plural, why does the sentence use the singular la trompeta?

In Spanish, when referring to individual items that each person in a group has or uses exactly one of, you typically use the singular. It behaves similarly to body parts or clothing. Using the plural las trompetas here could imply that each person in the group is trying to play multiple trumpets at the same time.

What would this sentence look like if it were a general prohibition for everyone, rather than just for us?