I think, that you must pronounce this word like "Subo", because many same words which describing "sleep" wrote with that letter S : Sleep, somnum, Сон. The cube mean Quadro , because the letter K (we pronouncing word CUbe throught this letter Kube) has numerical value 4, which creating the sQuare.
This is if we talk about words etymology, from where it came. But actually, if you fall down into the imagination and letter's meaning, you will see that the letter's C,K mean the same things. Everything in our life which chemically devided has the sleeping form, Hydrogen is sleeping form of energy, Devided peoples dna (chineses, blacks) are sleeping forms of human's...
Our ancestors described this imagination of sleeping just throught one letter - S, the letter S is analogy of Z, if the Z is awakness, life , the letter S mean sleeping and death. That's why the Russian word death "Smert'" wrote with letter S in beginning.