Here is what I would do.

You have everything good up until you need to play the 2nd sound. Could you just duplicate the current slide (in slide show mode you wont even know the slide number advanced if you use no transition animation) then set up your sound just like you did for sound 1 because you know that allready works. If this way will absolutely not work and you have to keep your slideshow to a certain maximum of slides you could put in a autoshape (off of the slide so it cant be seen) and put in a custom animation when clicked have it be something that will only stop when clicked then put your sound after that then you could pull it off.

Let me know if this still dosnt work there may be some more options.