A system comprising: an earphone comprising: a loudspeaker; a microphone; a housing supporting the loudspeaker and the microphone; and 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; a second earphone comprising: a second loudspeaker; a second microphone; a second housing supporting the second loudspeaker and second microphone; and a second ear tip surrounding the second housing and configured to acoustically couple both the second loudspeaker and the second microphone to a second ear canal of the user, and to acoustically close the entrance to the user's second ear canal; and a processor configured to: receive first input audio signals from the microphone; receive second input audio signals from the second microphone; detect peaks having a frequency of around 1 Hz in the input audio signals by combining the first input audio signals with the second input audio signals, and detecting peaks within the result of the combination; based on the detected peaks, compute an instantaneous heart rate; measure a frequency of an oscillation within the instantaneous heart rate; based on the frequency of the oscillation, compute a rate of respiration, wherein the processor is further configured to adjust output audio signals based on the rate of respiration, and provide the adjusted output audio signals to the loudspeakers. an earphone comprising: a loudspeaker; a microphone; a housing supporting the loudspeaker and the microphone; and 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; a loudspeaker; a microphone; a housing supporting the loudspeaker and the microphone; and 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; a second earphone comprising: a second loudspeaker; a second microphone; a second housing supporting the second loudspeaker and second microphone; and a second ear tip surrounding the second housing and configured to acoustically couple both the second loudspeaker and the second microphone to a second ear canal of the user, and to acoustically close the entrance to the user's second ear canal; and a second loudspeaker; a second microphone; a second housing supporting the second loudspeaker and second microphone; and a second ear tip surrounding the second housing and configured to acoustically couple both the second loudspeaker and the second microphone to a second ear canal of the user, and to acoustically close the entrance to the user's second ear canal; a processor configured to: receive first input audio signals from the microphone; receive second input audio signals from the second microphone; detect peaks having a frequency of around 1 Hz in the input audio signals by combining the first input audio signals with the second input audio signals, and detecting peaks within the result of the combination; based on the detected peaks, compute an instantaneous heart rate; measure a frequency of an oscillation within the instantaneous heart rate; based on the frequency of the oscillation, compute a rate of respiration, receive first input audio signals from the microphone; receive second input audio signals from the second microphone; detect peaks having a frequency of around 1 Hz in the input audio signals by combining the first input audio signals with the second input audio signals, and detecting peaks within the result of the combination; based on the detected peaks, compute an instantaneous heart rate; measure a frequency of an oscillation within the instantaneous heart rate; based on the frequency of the oscillation, compute a rate of respiration, wherein the processor is further configured to adjust output audio signals based on the rate of respiration, and provide the adjusted output audio signals to the loudspeakers.