Craft a musical composition that adheres to the specified musical theme and letter-based structure.
X:1
L:1/8
M:3/4
K:D
['dB g2 e2', 'Ad e2 a2 af ge fd']  
X:1
L:1/8
M:3/4
K:D
 dB g2 e2 | dc dA FD | dB g2 e2 | dc d4 :: Ad e2 a2 | af ge fd | Ad e2 a2 | af ge fd | dB g2 e2 | 
 dc dA FD | dB g2 e2 | dc d4 :|  