ALL AUDIO BELONGS TO DISNEY AND COMPOSED BY BRAD BREECK.
In case you are wondering, this is taken from Brad Breeck's Soundcloud. Make sure to head there for more Gravity Falls soundtracks!
Link:   / bradbreeck  
Follow Brad on Twitter:   / bradbreeck  

Presenting... The FULL score from Weirdmageddon III: Take Back The Falls! Make sure to save it into your device and ENJOY! :)

Some of the best parts
Gravity Falls Rock Theme: 10:32
Bill Cipher's Death: 24:59
Farewell: 34:37