There are some circumstances where only silent prayer is appropriate, e.g., praying for something between you and God only, praying for someone who is present, etc. There is not anything wrong with praying silently, as long as we are not doing it because we are embarrassed to be seen praying.
Kuna baadhi ya mazingira ambapo sala ya kimya ni sahihi, kwa mfano, kuomba kwa ajili ya kitu kati yako na Mungu, kuombea mtu ambaye ako hapo, nk hakuna kitu kibaya na kuomba kimya kimya, kwa muda mrefu ili mradhi tu kama hatufanyi hivyo kwa sababu tunaona aibu kuonekana kuomba.