A system comprising: an earphone (100;200) comprising: a loudspeaker; a microphone (106;206); a housing supporting the loudspeaker and the microphone; an ear tip surrounding the housing and configured to acoustically couple both the loudspeaker and the microphone to an ear canal of a user, and to acoustically close the entrance to the user's ear canal; and a processor configured to: provide output audio signals to the loudspeaker; receive input audio signals from the microphone; extract a rate of respiration from the input audio signals; adjust the output audio signals based on the extracted rate of respiration; and provide the adjusted output audio signals to the loudspeaker, wherein: to provide the output audio signals to the loudspeaker comprises to provide signals which represent sounds across a first frequency band, the audio signals including a notch in which the sounds lack energy within a second frequency band narrower than the first frequency band; and the processor is configured to extract the rate of respiration by applying a band-pass filter to the input audio signals to limit the input audio signals to a third frequency band contained within the second frequency band; and demodulating the filtered input audio signals to compute a rate of respiration corresponding to energy in the input audio signals in the third frequency band. an earphone (100;200) comprising: a loudspeaker; a microphone (106;206); a housing supporting the loudspeaker and the microphone; an ear tip surrounding the housing and configured to acoustically couple both the loudspeaker and the microphone to an ear canal of a user, and to acoustically close the entrance to the user's ear canal; and a processor configured to: provide output audio signals to the loudspeaker; receive input audio signals from the microphone; extract a rate of respiration from the input audio signals; adjust the output audio signals based on the extracted rate of respiration; and provide the adjusted output audio signals to the loudspeaker, a loudspeaker; a microphone (106;206); a housing supporting the loudspeaker and the microphone; an ear tip surrounding the housing and configured to acoustically couple both the loudspeaker and the microphone to an ear canal of a user, and to acoustically close the entrance to the user's ear canal; and a processor configured to: provide output audio signals to the loudspeaker; receive input audio signals from the microphone; extract a rate of respiration from the input audio signals; adjust the output audio signals based on the extracted rate of respiration; and provide the adjusted output audio signals to the loudspeaker, provide output audio signals to the loudspeaker; receive input audio signals from the microphone; extract a rate of respiration from the input audio signals; adjust the output audio signals based on the extracted rate of respiration; and provide the adjusted output audio signals to the loudspeaker, wherein: to provide the output audio signals to the loudspeaker comprises to provide signals which represent sounds across a first frequency band, the audio signals including a notch in which the sounds lack energy within a second frequency band narrower than the first frequency band; and the processor is configured to extract the rate of respiration by applying a band-pass filter to the input audio signals to limit the input audio signals to a third frequency band contained within the second frequency band; and demodulating the filtered input audio signals to compute a rate of respiration corresponding to energy in the input audio signals in the third frequency band. applying a band-pass filter to the input audio signals to limit the input audio signals to a third frequency band contained within the second frequency band; and demodulating the filtered input audio signals to compute a rate of respiration corresponding to energy in the input audio signals in the third frequency band.