I need help on directionality of speaker cables

The arrow points in the direction of current flow

Since current flows in both directions (AC signals) then I suggest that the arrow points towards something else (perhaps the person who fell for what must essentially be a marketing ploy)

My understanding was that audio signals could be carried as DC, but that AC was required for speakers to allow them to vibrate in piston fashion.

They could be carried as DC but they are not (in fact the sound you hear is alternating compression and dilatation of the air in your room....there is no stream of particles reaching your ears...the air just jostles around and you pick up these vibrations).

DC is used in power supplies but 99.99% of everything that involves information flow uses either AC or EM waves. Digital signals are transmitted in much the same way as analog but at much much higher frequencies...some kind of an alternating waveform (modulated by digital information to create a set of clealry distingusihable digital states that can be decoded at the receiveing end into the same stream of mathematical bits).

he reason I ask is that a digital signal, by it's very nature, doesn't have frequency in the way analog signals do. As I've quoted above from a reputable book on electronics, a digital waveform is a DC signal that varies between zero volts and a max volts,

Transmission of digital data, as I quoted from the electronics text, is DC

You are probably confusing the way Digital is represented in a drawing....most encyclopedia's and layman reference material incorrectly represents digital. When you see a typical analog waveform drawn with a "digital" stair steps superimposed that look like DC levels.... this is WRONG it doesn't exist like this.

Sorry but I studied digital signal processing and analysis in college...the whole subject has been badly dumbed down and leads to much confusion and fear of digital ( obviously a stair case is nothing remotely close to an analog waveform and hence the source of some of the malicious rumours about digital )

Digital data is sent as analog waveforms. Toslink Light is an alternating waveform as is the signal on an RCA coax or an HDMI cable. Typically all electronic devices used some form of MODEM between digital devices (Modulator and Demodulator; the digital data is encoded into some form of alternating signal often with a separate or embedded clock alternating signal to help in decoding the "states" into discrete "bits").

I've just asked one of my colleagues in the same firm who is an EE and without any hesitation said that it's DC in digital.

He needs to go back to school. Digital data is usually sent with an encoding such as Biphase Mark Code. This essentially amounts to a high frequency alternating waveform that travels down a wire (in which the clock frequency is embedded or can be recovered upon decoding). DC is used in all digital and electronic power supplies but not for transmission on a cable or through the air.