Any possible difference in sound between Toslink cables is in amount of jitter they produce. Output LED diodes are slow. Transitions in Toslink are about 10x slower in comparison to coax. On the receiving end of the Toslink weaker light will result in crossing threshold point at the wider angle. When signal is crossing threshold level slower then any system noise will alter exact moment of logic level recognition more - resulting in time jitter. With exactly square pulses there would be no difference between optical cables. Jitter translates to added noise that, by making signal sharper/brighter, can create illusion of increased volume.
Toslink
Does it make any sense that substituting one toslink cable for another, where the 2 toslink cables come from different manufacturers, could result in different volume levels?