Audio will only work one track at a time which is annoying as we can't do the thing of letting people make their own mixes of music things. However we CAN do it with films so the answer is to put the audio separate tracks on YouTube with an image to make it into a film. The track/film will always play from where you stopped  and of course you can only click on one at a time. Also we can make films which run concurrently and make it look like,for instance, we are walking from one situation into another...or things are moving from one film to another film e.g. Nicky's billiard ball films...