Da Voismart Knowledge Base.
Background a file with talk detect

  BackgroundDetect(filename[|sil[|min|[max]]]):  Plays  back  a  given
filename, waiting for interruption from a given digit (the digit must
start the beginning of a valid extension, or it will be ignored).
During the playback of the file, audio is monitored in the receive
direction, and if a period of non-silence which is greater than 'min' ms
yet less than 'max' ms is followed by silence for at least 'sil' ms then
the audio playback is aborted and processing jumps to the 'talk' extension
if available.  If unspecified, sil, min, and max default to 1000, 100, and
infinity respectively.