Develop a musical piece incorporating the designated alphabet-based pattern and the supplied melodic theme.
X:1
L:1/8
M:12/8
K:G
['BGB ADA', 'gab agf efg dcB Bcd efg']  
X:1
L:1/8
M:12/8
K:G
 c | BGB ADA BGB ADA | Bcd edc Tc3 B2 c | BGB ADA BGB ADA | Bcd cBA G3 G,2 :: gab agf efg dcB | 
 Bcd efg TB3 A3 | gab agf efg dcB | Bcd efg TB2 A G3 :|  