Mobile data processing Systems (MSs) interact with systems in their vicinity, and with each other, in communications and interoperability. Data is communicated using sound waves to carry out novel Location Based eXchanges (LBX) of data for new mobile applications. Information which is transmitted inbound to, transmitted outbound from, or is in process at, a mobile data processing system, is used to trigger processing of actions in accordance with configurations, for example to present content to a user.