There's a file in "Nostalgia Op2\PAN-2019112700\contents\data_op2\sound" called "music_list.xml", replace it with the attached one.
If the file is down, what I did to unlock the songs was simply to go to the voice "<unlock_type __type='s8' >[NUMBER HERE]</unlock_type>" of all the songs in the list, and replaced the number with "1", which seems to be the number chosen by the programmers for any song freely available from the start.
Use notepad ++ to do the modification
music_list.xml