You could either fade them in or, if you wanted an abrupt transition, use the <<timed>> macro.
Example: Fade in over 10 seconds
<<audio "trackname" volume 0 fadeoverto 10 1>>
Example: Playback which starts after 10 seconds
<<timed 10s>><<audio "trackname" volume 1 play>><</timed>>
SEE: <<audio>>, <<timed>>