Merleau-Ponty thinks that we perceive the meaning of the object, where the senses communicate with each other. Through this communication, we hear the sounds with tactile qualities ("soft sound") or we see the objects with softness, warmth or weight ("fragile looking glass"). He says;
By opening up to the structure of the thing, the senses communicate among themselves. We see the rigidity and the fragility of the glass and, when it breaks with a crystal-clear sound, this sound is borne by the visible glass. We see the elasticity of steel, the ductility of molten steel, the hardness of the blade in a plane, and the softness of its shavings....In the movement of the branch from which a bird has just left, we read its flexibility and its elasticity, and this is how the branch of an apple tree and the branch of a birch are immediately distinguished.... Likewise, I hear the hardness and the unevenness of the cobblestones in the sound of a car, and we are right to speak of a "soft," "dull," or "dry" sound....If they are taken as incomparable qualities, then the "givens from the different senses" belong to so many separated worlds - each one, in its specific essence, being a manner of modulating the thing - then they nonetheless all communicate through their meaningful core.
[Merleau-Ponty, M. (1945/2012). Phenomenology of Perception. New York: Routledge. p.238-9]
This description is true, but as far as it tries to describe the synesthetic nature of our ordinary perception. But I don't think its true, if it tries to explain the experiences of synesthesia reported by innate synesthetes. Their synesthesia does not seem to be mediated by any meaning but is the direct perception of the objects. The letter 5 looks green, the phone rings sky blue!