Develop a musical piece using the given theme and incorporating the letter-based structure provided.
X:1
L:1/8
M:6/8
K:D
['dff fdf faa afa adb', 'afg adb']  
X:1
L:1/8
M:6/8
K:D
|: A | dff fdf | faa afa | adb adb | agf f2 e | dff fdf | faa afa | adb agf | fee e3 :: A | 
 efg gag | gag Tg3 | fga aba | aba Ta3 | afg adb | afg adb | agf edc | ded d2 :|  