It recently happened at a wedding that when the groom performed the Kiddushin – placing the ring on the bride’s finger – the microphone stopped working, and the witnesses thus did not hear the groom’s declaration of "Hareh At Mekudeshet Li Ba’Taba’at Zu Ke’dat Moshe Ve’Yisrael." Although they saw the groom place the ring on the bride’s finger, they did not hear him recite these words, proclaiming that he gives the ring for the purpose of betrothal. The question thus arose as to whether or not the Kiddushin was valid, as the witnesses did not hear the groom’s proclamation. Clearly, the witnesses should optimally be positioned close enough to the bride and groom so they could hear him speak, but the question arises as to whether the Kiddushin is valid after the fact if the witnesses could not hear the declaration of "Hareh At Mekudeshet Li…"
The answer emerges from a Halacha that applies in a case of "Asukim Be’oto Inyan" – where a man and woman are discussing their plans and arrangements for marriage, and in the middle of the conversation the man gives the woman a ring without saying anything. The Halacha in such a case is that the betrothal is effective even though the groom did not explicitly declare that he gave the ring for the purpose of Kiddushin. Since they were discussing their plans for marriage, it is evident that the groom gave the ring for the purpose of Kiddushin. Undoubtedly, this would apply under the Hupa, as well. If a groom is dressed in tuxedo, the bride is wearing a wedding gown, and they are standing under a canopy in a banquet hall filled with guests and relatives who received invitations to the affair, it is readily obvious that when the groom gives the bride a ring, he does so for the purpose of Kiddushin. This certainly qualifies as "Asukim Be’oto Inyan." As such, even if the witnesses did not hear the groom explicitly state that he gives the ring for the purpose of Kiddushin, the Kiddushin is valid since the witnesses saw the transfer of the ring and it is abundantly clear that it was done for this purpose.
Summary: The designated witnesses at a wedding should preferably stand close to the bride and groom when the groom places the ring on the bride’s finger so they can hear the groom declare, "Hareh At Mekudeshet Li…" However, if they did not hear this declaration, the Kiddushin is nevertheless valid, as long as they saw the groom place the ring on the bride’s finger.