Craft a musical composition that adheres to the specified musical theme and letter-based structure.
X:1
L:1/8
M:6/8
K:Bb
['D D2 G G2 A B3 (c2']  
X:1
L:1/8
M:6/8
K:Bb
 D | D2 G G2 A | B3 (c2 B) | A3 B2 A | D3 z z D | D3- D2 d | d3- d2 D | (G2 A) B2 c | d3 (B2 c) | 
 d2 d B2 d | c2 c A2 c | B2 B (AG)A | D3 (B,C)E | G2 B A2 G | D3 C2 D | G3 G2 F | G3- G2 |]  