Audio

Audio signals can move through a very wide variety of media:

closed circuit
AM radio and FM radio, including low-power microradio
shortwave radio
walkie-talkie protocols and cordless protocols (such as PCS)
telephones
cell phones
web browsers and plug-in media viewers, e.g. for IP streamed radio
VHF and UHF (television bands)

delivery of signal

They are typically delivered to the customer's ears by the following means:

speakers installed anywhere using any of the above to receive a signal
headsets hooked up to any of the above.

ubiquity of transceivers

There are literally tens of billions of receivers capable of transmitting audio signals. This compares quite favourably to the number of Bluetooth devices. It would take an overwhelming argument for the absolute necessity of text or video signals to make a service that required them for Consumerium buying signal desirable. Historically, one could note that industrial radio was quite feasible and in wide use before the 1920s, while commercial radio was in wide use in the 1930s. However, the addition of video for television did not occur in developed nations until the 1950s, and the further addition of point-to-point still image and text communication wasn't possible until fax and email in the 1980s. And even by the 1990s it was not common to have devices capable of a wide variety of signals.

central role in culture

Psychological studies have repeatedly proven that the quality of audio has very strong effects on people's receptivity to information. Good audio even causes them to perceive the video as better than it is! Accordingly, all standards for Laserdisc, DVD and other media have put audio as the most important of their performance requirements.

Perhaps most important, anyone can speak, while not everyone can read or write, and not everyone can type, and certainly not everyone can draw good pictures. All civilization is based on poetry, song, myth, which are always most effectively delivered in audio form.

For more on this, and a vivid image of how important the voice actor can be, we recommend Neal Stephenson's book The Diamond Age which in part outlines the effect of a voice-acted interactive book called the Young Lady's Illustrated Primer. The Metaweb project is actually an overt attempt to move wiki in this direction! In the book, this voice acting literally pulls a youth at risk into a culture that is able to welcome, sustain, and support her.

We hominids are evolved to respond to our ape mothers' touch and sound. We are not evolved to respond to her text or beauty. Audio signal delivery is thus expected to remain the critical influence on most customer's buying decisions. This includes of course the word of mouth that is the most important influence on such decisions.