Every time I have a repeating exercise they always write 'we can't hear you' ?
to check that its not your fault: click "recording" and then, quickly, the play button (to listen to the computer voice). when the voice has finished, stop recording. if it does not accept the answer, listen to the playback and, if necessary, adjust distance and direction speakers - mic.
My answer is "it's broken". I'm not sure why but audio input is now broken. I have always used Firefox but this didn't work on a new computer. I have tried this now with Firefox, Opera and Konqueror, on Kubuntu and Ubuntu 12.04 including going back to an older computer where it had worked. Maybe the issue is loading stuff from a cloud that used to be on duolingo?