Fahnsen:
Gentlemen, this is digital signals, consisting of ones and zeros, that is ons and offs. There is no way known to science why the sound should alter with different cables, or with electrical versus optical signals.
As well as sending 1s and 0s you have to take into consideration the timing of the sending of the 1s and 0s, which is jitter. If something goes wrong with the 1s and 0s you will hear it as the music will cut out or crack or pop. So really the issue is about jitter and the timing of the signal. Whether a cable is cable of reducing jitter, I am not sure. But different cables may well be better at not introducing more jitter.
An argument often raised questioning the validity of different cables is that when sending a document to a printer you get the printed document as it should be and not with random faults to the print. However, the lack of faults is down to the lack of importance for timing of the sent data. So long as the correct data is sent there is plenty of time to set up the printer etc. So jitter is not a factor, so the cable is not a factor.
When you send a digital signal a clock in the sender, say it is a PC determines the timing of the 1s and 0s. The receiver, say it is a DAC has a clock to time in the 1s and 0s. To work both clocks have to run at exactly the same time. However, (and I have read up on this and I am not an engineer) it would appear that getting those clocks to work precisely is very difficult if nigh on impossible.
When listening to music, the timing is important to get the right sound. Then, there is a lot more going on when sending 1s and 0s to make music than to print a document, so in this application jitter is important.
Better clocks and use of buffering the data are ways of actually reducing jitter as it is sent from PC to DAC. So what we need to do is decide can a cable effect jitter? Then we have an answer as to whether digital cables are capable of being different or not. We also have an answer as to why sometimes people report a difference between cables and others do not. If you introduce a cable to a system and it does not affet the jitter, there is no reason why it should sound different. But if you do introduce a cable that affects the jitter, this is now a reason why it can make a difference.