"But what happens when the sound you want to search happened five seconds ago?"
someone else already suggested they add an option for that "feature". You know, on, pause, off; or perhaps, on, off, "no, seriously off". whatever.
And the audio may not be tracked by *THEM* but a back door trojan horse application *COULD* perhaps 'hook' it and leverage the user's cluelessness with respect to having the microphone on whenever that application is left running...
[maybe we can blame the "TSR"-ness of phone applications, too? Must they REALLY 'stay running' all of the time?]