Fortunately, I managed to find a Japanese wiki that has the information I need: http://www37.atwiki.jp/konpsp/pages/27.html ^^;
It seems I need to play a lot of Fuwa Fuwa Time song. I don’t remember having much trouble with this song, which is probably why I never retried it enough times to get the random voices unlocking.
I decided to work with Mio first since she only had 2 voices left.
On my third try, I unlocked Mio’s #46 voice:
And on my eleventh try, I unlocked her #15 voice:
- Yui: 16, 53, 54 (3 voices)
- Mio: COMPLETE
- Ritsu: COMPLETE
- Tsumugi: 16, 48, 49, 50, 51 (5 voices)
- Azusa: COMPLETE
- Total: 8 voices
Maybe I should take my time and unlock a voice a day… ^^;