elapsed time would be cool...

[no-stack-dub-sack]

...to show the relative efficiency of each sorting algo.

This is an awesome project though! And if you don't mind, I may add it as a resource to my project https://cs-playground-react.surge.sh to help people using it to visualize sorting algos. The only similar tool I've ever come across, albeit without audio (which also may help you figure out #4 as they do have mergesort) is https://www.cs.usfca.edu/~galles/visualization/Algorithms.html

EDIT: actually, just came across this as well, which if you've ever googled sound of sorting you've probably seen, but if not, there may stuff useful to you here as well (though a web version of this is much better!) http://panthema.net/2013/sound-of-sorting/