This is one example of the fact that Chinese etymology can be traced back much further to roots and sources than Indo-European etymology goes. We don’t know why tree and three rhyme, but we do theorize that 木 and 目, which are sound-alikes are so called, perhaps, because to the ancients there were trees as far as the eyes could see. 相 and since tree and eye are sound alikes put them together as the symbol for “alike”/”similar”.
There are many other examples of words in Chinese where we know that figurative uses arose out of literal things. I don’t see that level of specificity in origins of Indo-European words. From Chinese etymology however we can and should hypothesize that sound-alikes are figurative extensions from literal uses of the same term. Thus it is a valid hypothesis that mutter and mother are related words in that only your mother would understand whatever you mutter, e.g.
The more I study Chinese the more I beleive in proto-world.