first of all, i never used sound, so i'm not an expert, i just try to answer till Luis has some time to reply
1. for me, it's not obvious, what do you mean about "my sound folder". there is a main sound folder in the root of the game, and each efiles can have their own local sound folders. the local folder is checked first. see the
"music" section on Luis' site. be sure you use your local efile folder for customization
2. it's also not clear for me, what do you mean about "when I replace it": do you overwrite the gplay1.mp3 by another file having the same name, or do you edit the scenario in suite or something else?
3. i checked the music file selection in suite, and it works for me
are you sure, your music files are placed into the local sound folder? are these files readable? where is your opengen installed? there are some folders what shouldn't be used, see this hint from our site:
When selecting the installation folder, you should avoid special Windows folders like “Documents”, “Programs Files”, “Users” or similar, as later you won’t be able to add new stuff or even save your savegames. The recommended installation folder is “C:\OpenGen”.
sorry, that's all what came into my mind, i hope you can figure out what went wrong for you