1. A method of detecting failure of an acoustic echo cancelling component in an endpoint device, the method comprising: conveying a known audio signal i) to a loudspeaker communicably coupled to the endpoint device, wherein the loudspeaker outputs the known audio signal as sound, and ii) to a post-processing delay estimator; conveying a processed audio signal to the post-processing delay estimator, wherein the post-processing audio signal is generated by the acoustic echo cancelling component in response to a microphone audio signal received by the acoustic echo cancelling component from a microphone communicably coupled to the endpoint device; calculating, by the post-processing delay estimator in response to the known audio signal and the processed audio signal, an estimated post-processing delay that is an estimate of an amount of time between conveying the known audio signal to the loudspeaker and outputting of a portion of the processed audio signal that includes the known audio signal from the acoustic echo cancelling component; calculating a confidence level for the estimated post-processing delay, wherein the confidence level for the estimated post-processing delay is a probability that the estimated post-processing delay is an actual amount of time between conveying the known audio signal to the loudspeaker and outputting the portion of the processed audio signal that includes the known audio signal from the acoustic echo cancelling component, and wherein calculating the confidence level for the estimated post-processing delay includes: calculating a weighting value based on a variance of the estimated post-processing delay, and calculating the confidence level for the estimated post-processing delay as a product of the weighting value and a fixed value; comparing the confidence level for the estimated post-processing delay to a minimum confidence level threshold; and in response to detecting that the confidence level for the estimated post-processing delay exceeds the minimum confidence level threshold i) detecting that the acoustic echo cancelling component has failed, and ii) storing an indication that the acoustic echo cancelling component has failed in a memory of the endpoint device.