This solution originates in the sigma delta modulation commonly used for digital to analogue converters or already for control in Digital Current Control Using a Switching Power Amplifier and Sigma Delta Modulation. The basic idea is to employ the sigma delta modulation principle to directly drive a power switch or a H-bridge near the actuator coil of the loudspeaker.
Imagine a tristate sigma delta modulator is running at a certain speed, let's say 700kHz, and acts directly on the digital PCM signal comming from the CD, or as analogue device on the analogue signal. Of course the signal quality will depend on the oversampling rate and hence the resulting switching frequency, which has to be about 3MHz to achieve the audio quality. The so generated digital signal now drives a power switch, a H-bridge or any switching power device (a very fast relay with recovery diodes). The signal level is controlled by the voltage switched onto the inductive load, i.e. the loudspeaker which is a low pass.
The digital audio amplipfier can be devided into two parts : a digital modulator with power supply offering two outputs, a coaxial output to the loudspeaker conveying the signal and a voltage supply, respectively. The second part, the switching device can be directly built into the loudspeaker. In any case this could be a cheap solution for larger and/or remote systems as for airports or train stations. Oversampling a 8kHz 8 bit ISDN signal by a factor of 64 yields 512kHz which seems to be easy to realize at reasonable costs. A MATLAB/Simulink implementation visualizs the signal processing steps.
Note that the sigma delta modulation and upsampling takes place in the digital domain, so no analogue signals are involved. Using off the shelf power electronics and switching technologies, this solution could be scaled for any audio quality and requirement.