For the song files in the Super CD+G format (disc or drive), the index number of the song files do not change. But for the song files of MP3+G or AVC in a drive, the index number of the song files can change when song files are added or removed. It is because the index numbers are generated each time according to the order of the song files being copied into the drive.
To fix the index number of the song files from changing, name each of the song file as #00000-TITLE-SINGER.
For example,
#12345-TITLE-SONGER.MP3, #12345-TITLE-SINGER.CDG
#12345-TITLE-SINGER.AVC
The song files named as above will have the index number fixed as 12345.