Develop a musical piece incorporating the designated alphabet-based pattern and the supplied melodic theme.
X:1
L:1/8
M:6/8
K:G
['(B/A/) GFE FGB AGE']  
X:1
L:1/8
M:6/8
K:G
 (B/A/) | GFE FGB | AGE E2 (G/A/) | BAG AGA | BGA B2 G/A/ | BAG AGA | BAG A2 (B/A/) | GFE FGB | 
 AGE E2 ||  