Congratulations ! Here is the flag: PaxH7".aoD10+a

The audio file also hides 156 successive consumption curves, that you might already have found. The plaintexts used for the generation of these curves are from http://www.maryjones.us/ctexts/a01w.html, without any caps or new lines, encoded in ASCII. 156 blocs of this poem were used (from I to X). As an example, here are the first 25 blocs:

gredyf gwr oed g
was gwrhyt am di
as meirch mwth m
yngvras a dan vo
rdwyt megyrwas y
sgwyt ysgauyn ll
edan ar bedrein
mein vuan kledyu
awr glas glan et
hy eur aphan ny
bi ef a vi cas e
 rof a thi gwell
 gwneif a thi ar
 wawt dy uoli ky
nt y waet elawr
nogyt y neithyaw
r kynt y vwyt y
vrein noc y argy
urein ku kyueill
t ewein kwl y uo
t a dan vrein ma
rth ym pa vro ll
ad un mab marro
kayawc kynhorawc
men y delhei dif