how the PCM works?

Question

Explain how the PCM works. Why is it enough to receive a transmission of at least 56kbps to listen to music over the Internet? Does this transmission rate violate Nyquist theorem? Justify the answer. the bandwidth of a public telephone line is about 4KHz

Answer 1

PCM stands for Pulse Coded Modulation.

You appear to be confusing it with mp3 compression.

PCM is uncompressed sound data.
Each sample's amplitude is measured and stored as a number.

Compression takes these numbers and squeezes out redundancy. There is lossy and lossless compression available.
The upper frequency limit of the
telephone network is now commonly accepted to be about 3.3 kHz at best. The last predivestiture
Bell PSTN tests in 1984 showed significant rolloff at 3.2 kHz for short and
medium connections, dropping to 2.7 kHz in long distance connections2. At the low end
of the spectrum, the telephone network carries frequencies no lower than 220Hz, and
most commonly only as far down as 280 or 300 Hz.
The Nyquist limit is that at least two samples are needed for every hz. of analog bandwidth. It does not work the other way around because the data is compressed and therefore there can be many more zero transitions at the speaker once the redundancy is reproduced by data decompression.
The analog bandwidth is different from the digital bitrate because many digital bits can be encoded onto a single analog wave shape.