The verb "a trebui" is impersonal and to conjugate it in the indicative present form you just use "trebuie" for all the persons in singular and plural. You only have to pay attention to the verb following "trebuie", because that should be conjugated according to the subject. In this case: (noi) să tăcem.
Thanks a lot. In which occasions are the other forms used (for instance noi trebuim)?
I don't remember hearing it. In theory it is not used, perhaps only in archaic contexts, but still it is incorrect.
My interpretation: "One must" encompasses both 3rd person singular (he/she/it) and 1st person plural (we). I'm not a native speaker, but this makes sense to me.
This is a difficult topic even for Romanians. You can find the forms of "A trebui" at conjugare.ro , yes it has personal forms such as "noi trebuim" but to use this form "we" must be the subject of the need. "Noi le trebuim lor" = "they need us". In 99% of the spoken language cases "trebuie" is impersonal and intransitive ("it is needed").
noi trebuim doesn't exist in Romanian...it's trebuie for all persons, both singular and plural!