What are the capacitors between the IN and OUT pins of a TDA7318 mixer chip for?
There is not much in the datasheet to work with, however, it is clear that the capacitor removes a DC path between two different internal modules of the chip.
Any DC bias that might get added on the input selector + gain block would pass on to the volume block, unless the capacitor is there to remove it so the volume block can set its own internal input bias voltage.
Since the manufacturer suggests this configuration, it would be more preferable than to do it against manufacturer suggestions, especially since the datasheet does not describe the internal operation in a detailed way to let the designer choose between the two options.