Several systems are case sensitive, so errors appear about this. Could you correct them please, thank you very much for your time, a great game!
i use linux .... Deepin 15.5
Heyho pancram!
Thank you for playing and for reporting the bug! 
Sadly, we won't be able to fix this error due to the contest rules this month (we're not allowed to upload newer versions as this would skew the competition). After this, one of the first things on our list!
Maybe this hot fix can help you play (quoting user DrkSky from below in the comments) :
"so ninja_0X.png files are being called improperly and throwing a load error.
So is the LetterSound_Neutral.ogg.
GNU/*nix players can manually rename the files changing the case as required, untill a fix is released. A quick enough rename."
Stay funky!
Dez