Yes, it means, "be good." However, was the speaker talking to two or more girls/women, with no boys/men present? If there was a crowd of girls, even if there was but one man, the sentence would be "soyez bons." Hmm, you say this was heard at the end of a speech. If the speech was addressed to a large crowd, how likely would it be that the crowd was composed entirely of women? (to warrant the use of bonnes instead of bons). "Soyez" looks exactly like the subjunctive, but it is actually the imperative in this sentence. The two forms (subjunctive and imperative) are identical for the second person plural (or second person singular/formal).

ok, quite old fashioned for my taste ! Its not subjonctive because subjonctive NEEDS "que + pronoun" (ex : il faut QUE VOUS soyez bons"). Verbs without pronouns in the imperative form (it's an order). For être, it's "sois, soyons, soyez" for imperative. So, yes, it means "be good". (Notice that when we French speaker use imperative, you often use infitive in English.)

Allez craig, sois bon !!! :)

Headmaster addressing a group of students about their upcoming school tour. He was telling them what they would be doing then said 'Soyez bons' at the end.

it's an odd thing to say, in a speech or elsewhere. What was the speech about?