Well replacing the songs (as an example in Non-Stop-Pop) is a little more complicated than deleting all and putting in mp3.
First you should restore the original file (the rpf for the pop radio).
Then you navigate inside the the rpf and select a song you like to replace. (Example: "Don't wanna fall in love")
Right-click and select the "Export as Open Formats"
No select where to save the exported file. > This will create a folder with the name of the Song and an "oac" file
Now inside the folder are two wav files (one is the right and the other is the left side)
(Sidenote: I always open Audacity for this)
Now open the left side wav. with your preferred Audio-Editor.
Replace the Audio with the left side track of the song you like (in this example should be around 3:21 in length)
After that repeat the same with the right side. (Be sure that you save on the exported file and that your audio editor keeps the audio preferences when clicking save, so you don't change hz, rates etc..)
After you did this with both the left and right wav files, you now have your "replace song" as two wav files for each side.
Now navigate back the oac file and drag and drop it back into the pop radio rpf in OpenIV. You should see a short "importing" window. After that you can check by previewing the replaced song in OpenIV.
To complete the replace, go to your language file (where the global.gxt2 is also located) and check for "trackid.gxt2"
Repeat the similar process as you did when renaming the self radio. Inside the track ID are the infos for the songs saved.
So search for the name (and artist) of the song you replaced and put in your replace song infos.
Save and re-import the file and now your should hear your own song in the regular GTA Radio and have the correct Artist-Name infos too.
If my text sounds too complicated for you, let me know and I try to make a short screengrab video of the process.
It may sounds hard at first, but once you get the hang of it, it's really easy.
I have many songs/stations replaced.
However I do advice against deleting any files, since that might interrupt flow or cause short (or long) silence and weird behavior.
As of now, I didn't find any settings details for the radio, so best is to replace songs in a 1=1 manner (no deleting or adding new), so you can get your own music into the game on more than just Self Radio
Of course, you should be sure to copy any radio you want to edit to your mods folder first, so it keeps and don't get overwritten by updates or similar