In Isaiah 55:11 God says, "My word which goes forth from My mouth; It will not return to Me void, without accomplishing what I desire, and without succeeding in the matter for which I sent it." Jesus said in Mark 7:13, "The traditions of men make void the word of God." So in one place God says His word can't be made void & in another He says it can, thus these must be talking about different "words."
God got the idea across in Isaiah 55:11 of, "I will accomplish what I desire." Something like that pertains to keeping promises. God makes a similar statement in Isaiah 45:23 while making a specific promise at the end, "I have sworn (made a promise) by Myself, the word has gone forth from My mouth in righteousness and will not turn back, that to Me every knee will bow, every tongue will swear allegiance." Remember also 2 Timothy 2:13, "If we are faithless, He remains faithful. He cannot deny himself."
Another interesting thing about Isaiah 55:10-11 is that it is actually future tense. He says, "the word from my mouth shall be as trustworthy as the rain coming from above to water the earth before it evaporates." Thus, at the very least, we cannot assume "the word" He is speaking about is scripture written before Isaiah.
In Mark 7:5-13 Jesus first quotes Moses (scripture) then He quotes a tradition which makes a truth claim. Here the "word of God" being referred to is the truth of God found in scripture. It can be made void by man made traditions.
The parable of the sower in Matthew 13 also gives us this idea of the word/truth of God being made void. There Jesus speaks of "the word of the kingdom" referring to the truth of the kingdom, something we now have written in scripture. Jesus says "the word can be snatched away by the evil one," "the word can fall away because of affliction or persecution," & that "the word can be choked & made unfruitful by the worries of the world & the deceitfulness of wealth."
So it is evident from the teachings of Jesus & the original context of Isaiah 55:11 itself, God saying, "My word will not return to Me void" does not refer to scripture or the reading of it but instead to the effectiveness of His promises.