I have this issue on and off with miui make sure theres no .nomedia files in the same folder as your music as it will be skipped if there is, also see what happens when you move the music into a folder called music on the SD card /sdroot/music/file.mp3 and see if it gets picked up, your music may just be too deep. you could also be facing an issue with the music being filtered by folder, have a look at the folder filtering settings in miui music you may have to select your music folder to filter/unfilter it from displaying.
