Increasing volume on usb output

[NormOnatorr]

Hi Chaps/Chapesses! 
If I may ask a quick question which I cannot seem to find anywhere:
Q: how do I increase the usb output from the RaspPi when using PiCore Player? I do not know if this is even possible but it is too quiet using an external Muse dac.
Apologies in advance if I am in totally the wrong place but I feel like I am banging my head against the wall now with this so any help gratefully appreciated! 
Cheers