Q: How do I increase the volume of Festival / decrease the volume of my
   MP3s so that I can hear the speech over the music ?

A: 1) Add the following to the end of your
   /usr/lib/festival/lib/siteinit.scm or /usr/lib/festival/siteinit.scm :

(set! default_after_synth_hooks
   (list
     (lambda (utt)
       (utt.wave.rescale utt 1.0 t))))