Put the code inside an alarm and inside each if for a song you also put a timer for the same alarm with the size of the song. I mean, it bit be kinda annoying but it's the simplest way.
Though I guess you can also use if !sound_isplaying(selected song) inside an if for the equivalent dice number for the song in the step event, that should prolly work too, but tbh that sounds like more work than just getting the times, personally :p
if dice == 0
{
if !sound_isplaying(paul_chuck)
{dice = round(random(6))}
}
Also I might be wrong but doesn't that code allow it for there to be repeats of the songs?