Also yes, I used the meow sound, but that's because it has a counterpart in scratch, which actually allows you to edit your sounds, so I can see if it's not to far from how it should work.
Downloading code won't work for my computer because I cannot see code files-
maybe you forgot a 
block?
I meant download the sound, by pressing the green down arrow button it downloads an mp3(you need to add the extension.)
What extension?
.mp3 to the end of the file.
It doesn't seem to work on the project page...
Also:
A few things here.
The parameter that will be passed is a list of files the user selected, not a single file.
You have to put 'item 1 of _' in there instead.
Also, it says to read the file as... text??? Why? Make it read sound!
Also, you should replace the place where it says .mp3 to say audio/*, so it will accept all audio files. The read file block can handle it.
??? What?
When I click it in the project page, it does nothing, but when I click it in the editor it downloads the file.
Also,
This is actually faster if you remove the wait .1 secs.
Also, you did forget a play sound block:
Replace the script in the green button with this.
Also:
Replace the part where it says 44100 with the block in its place in both the up and down arrow scripts.
Ok, I fixed it.
bump!
Sorry if this is annoying, but PLEASE REPLY, I still want to work on this. If you reply I know you like this project.
Why not just use the scratch sound editor or some other sound editor and import it back to Snap!?
I'm coding for fun, not making this for actual REASONS of course!
I made a version of what you have.
https://snap.berkeley.edu/snap/snap.html#present:Username=joecooldoo&ProjectName=Snap!%20Audio%20Player
I'm just getting a tone that's a B4 I think.
What is B4?
It's a piano note. (About 500 Hz or something)