A system to modulate packetized audio signals, comprising: natural language processor component executed by a data processing system to receive, via an interface of the data processing system, data packets comprising an input audio signal detected by a sensor of a client device; the natural language processor component to parse the input audio signal to identify a request and a trigger keyword corresponding to the request; a direct action application programming interface of the data processing system to generate, based on the trigger keyword, a first data structure representing an action responsive to the request, wherein generating the first data structure comprises selecting a template and populating one or more fields in the template with information detected by one or more sensors of the client device or obtained via a user interface of the client device; a content selector component executed by the data processing system to receive the trigger keyword identified by the natural language processor and select, based on the trigger keyword, a content item via a real-time content selection process; an audio signal generator component executed by the data processing system to generate an output audio signal comprising a first portion corresponding to the first data structure and generated with a first audio modulation, and a second portion corresponding to the content item and generated with a second audio modulation different from the first audio modulation, wherein at least one of the first portion and the second portion of the output audio signal is modulated by decreasing or increasing at least one of the frequency and amplitude of the output audio signal, or by applying an audio overlay or an audio watermark to the output audio signal; and an interface of the data processing system to transmit data packets comprising the output signal generated by the audio signal generator component to cause an audio driver component executed by the client device to drive a speaker of the client device to generate an acoustic wave corresponding to the output signal. natural language processor component executed by a data processing system to receive, via an interface of the data processing system, data packets comprising an input audio signal detected by a sensor of a client device; the natural language processor component to parse the input audio signal to identify a request and a trigger keyword corresponding to the request; a direct action application programming interface of the data processing system to generate, based on the trigger keyword, a first data structure representing an action responsive to the request, wherein generating the first data structure comprises selecting a template and populating one or more fields in the template with information detected by one or more sensors of the client device or obtained via a user interface of the client device; a content selector component executed by the data processing system to receive the trigger keyword identified by the natural language processor and select, based on the trigger keyword, a content item via a real-time content selection process; an audio signal generator component executed by the data processing system to generate an output audio signal comprising a first portion corresponding to the first data structure and generated with a first audio modulation, and a second portion corresponding to the content item and generated with a second audio modulation different from the first audio modulation, wherein at least one of the first portion and the second portion of the output audio signal is modulated by decreasing or increasing at least one of the frequency and amplitude of the output audio signal, or by applying an audio overlay or an audio watermark to the output audio signal; and an interface of the data processing system to transmit data packets comprising the output signal generated by the audio signal generator component to cause an audio driver component executed by the client device to drive a speaker of the client device to generate an acoustic wave corresponding to the output signal.