How to delay experiment end by varying times

In my experiment I am playing an audio story (in block) while the participant does a reaction time task. The visual stimuli which appears during the task is at random invtervals. The problem I am having is that when the RT task has concluded the experiment ends (or moves to the next block). I want to delay that so that the audio story goes all the way to its end, so participants hear the whole story, before they move on or exit the experiment. I can't use time-since-start since there will be multiple blocks, and I can't figure out how to use timestamps in a block. 

Any advice?

task RT

set $x random 10000 26000

delay $x

show bitmap Email 300 200

readmouse 1 1 3000

save STATUS RT BLOCKORDER


block DP_Nar_block

    sound DPNarr

    tasklist

        RT 9

    end