I would like to contribute to the replacement sounds and music, but I don’t have RPG Maker to access the RTP sounds, so I don’t know which sounds need replacing. Is there any way to find/play the original audio files otherwise?
On a related note, what is the best way to find which ones have already been replaced? (Apologies, I’m new.)
Hello @RegulusPastor, welcome aboard. 
Our RTP replacement really needs some good contributions, so we are happy you are here.
The official 2003 RTP is a free download, so you can extract it and have a closer look at the files: http://www.rpgmakerweb.com/download/additional/run-time-packages (click the black RM 2003 button)
Our replacement package is located here: https://github.com/EasyRPG/RTP
We use git version control system for it. Don’t worry, if you do not know how to use git (yet), we will also accept files send by mail and such.
We try to use meaningful names, most of the time the unofficial translation and official translation are similar, but not always. You can find out some mappings of the file names used here in our wiki: https://wiki.easyrpg.org/development/rtp-database/2003-rtp
Last but not least, feel free to ask questions!
We are also available on IRC btw.: freenode/#easyrpg (not 24/7 of course 
)
Have fun!