A transaction initialization system serving first and second devices, interconnected by a communication network, each having a speaker and a microphone, which wish to perform a transaction, the system comprising a server; and a client which controls the first device to initiate the transaction by (a) commanding the first device's speaker to generate a transaction-heralding sound and by (b) notifying the server, via said network, of generation of the transaction-heralding sound; and controls the second device, using a processor, to notify the server, via said network, that a particular transaction-heralding sound has been heard; and wherein, at least once, when the server notes that the same transaction-heralding sound has been reported as having been sent and as having been received, the server responsively validates a transaction between the two devices.