Da Voismart Knowledge Base.
Play an audio file while waiting for digits of an extension to go to.

This application will play the given list of files (do not put extension)
while waiting for an extension to be dialed by the calling channel. To
continue waiting for digits after this application has finished playing
files, the WaitExten application should be used. The 'langoverride' option
explicitly specifies which language to attempt to use for the requested sound
files. If a 'context' is specified, this is the dialplan context that this
application will use when exiting to a dialed extension.  If one of the requested sound files does not exist, call processing will be
    s - Causes the playback of the message to be skipped
          if the channel is not in the 'up' state (i.e. it
          hasn't been answered yet). If this happens, the
          application will return immediately.
    n - Don't answer the channel before playing the files.
    m - Only break if a digit hit matches a one digit
          extension in the destination context.