Quote from Nookies;1285619:
I heard about Fleta's secret shop after you get a empty treasure chest from bears is that sill available? :chomp2: I heard she then sells music scrolls which strongly give you instrument playing exp.
I have a lot of scrolls and for some reason I sometimes make a new scroll, play it and it tells me I am playing the same old song when I never played it before.
Rank up your music knowledge skill. It upgrades the difficulty rank of scrolls you compose so you can keep training playing better. Barry also offers "master" rank scrolls with his PTJ (high success rate and can perfect at any rank), but that only works for 2 perfects per game day.
The chest you mention is used to start a quest, whereby Fleta gives you Aeria's secret shop coupon, which has the music knowledge B->A book (as well as medi E->D). So yes, that chest is part of what you want, just not in the way you seemed to think.