Toslink

I can't think of a mechanism for a volume change with optical cable change, no. Doesn't mean its totally impossible though.

A parallel question: IS there any appreciable audible differences between Toslink cables?

Volume changes seem doubtful but clarity is another story. Some of us have found enough difference between glass and plastic to stay with glass. Also, my Lifatec cables handle 24/192 while the Blue Jeans cable (which is nicely made BTW) can only go as high as 24/96. Same with other plastic ones I've tried.

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.

op

glass fiber cables better than plastic element ones

search 'glass toslink digital cables' several sources will pop up, prices are reasonable (but more than the cheapie plastic core jobs)