I just bought this phone in australia from our largest carrier 'telstra' and i am sad to say that i love everything about this EXCEPT the sound!
Coming from the htc one x and my daily rig being the NWZ F808 paired with atrio m7s and a headstage amp i noticed that the Z1C not only lacks volume but also bass. The drums and bass lines play with little bass extension, there is virtually no energy in the bass notes with this device. I cant understand why, i haven't heard anything this ridiculous since, well ever. Ive checked all the settings and even used the my go to music app, Gonemad music player with all the equaliser settings adjusted to enhance the bass and volume, still, nothing... Its not just my device though just this model, i checked with all the demo devices at all my carriers stores and they all sound the same.
the app Viper4Android will solve your issue. must be rooted to use it tho