Craft a musical composition that adheres to the specified musical theme and letter-based structure.
X:1
L:1/8
M:2/4
K:C
['c2 ec cBAG', 'G3 A GFEF']  
X:1
L:1/8
M:2/4
K:C
|: c2 ec | cBAG | defg | fedc | c2 ec | cBAG | AcBd | c2 c2 :: G3 A | GFEF | GABc | defg | G3 A | 
 GFEF | AcBd | c4 :|  