I think it was not accepted because of the switched word order. In the sentence, Duolingo has «Per favore» at the beginning of the sentence and then «pulite». Also, in English, we do not use personal pronouns in imperatives. If we are giving a command we do not say, "Please you clean my room;" we say "Please clean my room," which is accepted.
P.S. Also, the imperative matches the present indicative conjugations for not just the «voi» form but also the «noi» and «tu» form, so «pulisci», «puliamo» and «pulite» can all be either the imperative or the present indicative tense: http://www.italian-verbs.com/italian-verbs/conjugation.php?verbo=pulire
If you start a sentence like this with "You all" then there is no point in saying please, it comes across as rude. I would probably phrase that "Could you guys please clean my room". The problem is if you use "you all" to start a sentence requesting a favour it holds a negative tone. You can also say "Can you clean my room, please" and the "you" in the sentence can mean depending on context "you all". It is generally a good idea to start polite requests with "Can you…" as this is not demanding. But don't say "Can you please…" (irritated tone), say "Can you …., please" (polite tone).
Sometimes the sound does not come through. If I recall, there is a 'button' in the lower left that says something like 'can't listen now.' It will not dock you for missing the question, but simply move on with questions that require no sound. Eventually, it will come back on. Sometimes I might simply get out of the app and relaunch it again and that often works. Also, there may be a 'skip' button in the lower right, so you can try that one too. The issue may be question specific, so that the next one may work okay.