Thats because mod verifies checksum of original game .pk3 files.

You need to create your own custom .pk3 and not modifiy original ones.

Oh. Is there a guide for how to make a pk3 that will override it then?

I found a guide for making pk3s on gamebanana and used it, but it doesn't seem to work.

.pk3 is just renamed .zip

It works when I load it from addons, but for some reason my game reverted back to the original RTCW menu without episode select or bonus difficulties, the mission briefings seem to be overlapping (2001 version and new version at the same time), and the hud no longer works, along with all of the weapon pickup names being wrong. Any ideas of what might fix this?

That means that there is something else in your .pk3 that doing this

I named the pk3 file z_realrtcw_enhancedmusic.pk3. It contains sound/music/, and in the music folder I only added the 6 songs I replaced. Is there anything else I should have in there so RealRTCW works correctly?

I have come to the realization that the actual location of the addons for RealRTCW is in my document's folder in RealRTCW 4.0 and not in my RealRTCW/Main folder. When I got the music addon working, it was in a folder called MusicMod with the sp_pak1.pk3 in it, which also caused the original glitch, but when I removed it and replaced it with my own pk3 it stopped playing the new music, but the menu glitch is fixed.

I actually got it working now, but I changed what I had for m_moving to a different song, and now it plays a high pitched static noise instead of the new song. Any ideas of what might cause this?

Wrong format

it's still a .wav, but I did convert it using Audacity, so there's a chance one of my Audacity settings breaks it.

I think it's the signed-24bit-PCM encoding that broke it, because the others were in the default 16 bit setting.

I got everything working perfectly. One more question: If I made a pk3 file and put it into Vendetta 3 for example, and it happened to successfully change the music of that mod, would there be any way of releasing it so it affects the other mod if the user has installed original? or would I need to release the original mod + my pk3, or ask the creator of the mod to add in my pk3 to a version of their mod in order for it to affect their mod?