This is a common question, that I've haven't seen answered.
If you re-import any sound file from the game's audio folder back into it through rmxp it corrupts and doesnt play, but you say it doesnt work with the rtp SEs either. This is a very common problem but sadly I've not known it to be answered. When I've had trouble with all kinds of sound files, BGM, ME, SE and BGS I did some strange Direct X sound check through some RUN thingy, I don't think that was the right fix for my problem though, I solved my problem by restarting the computer, you might want to look up that Direct X check thingy, search it in your search engine of choice with the words RPG Maker involved or even try restarting your computer.