how is digital sound created?

If the OP means electronic music or sounds, then I suggest looking up the history of digital synthesizers and FM synthesis.

At its most basic, sound (and music) can be modeled as the sum of sine waves of different amplitudes and frequencies. This is the most important concept to understand and underpins all of digital audio. Here are some links that you’ll hopefully find useful:

https://www.compadre.org/osp/EJSS/4487/272.htm

https://gizmodo.com/digital-music-couldnt-exist-without-the-fourier-transfo-1699155287

The only interpolation that is part of the digital audio standard is when it is used for error correction.

There’s no interpolation happening when performing error correction since there’s no ’guessing’. The proper bits are either recovered or the data stream is so corrupted that some errors remain. In the latter case, the player may mute the output or cease playback.

Interpolation is required whenever you increase the sampling rate in digital audio. Most DACs these days, whether some form of multibit resistor ladder or the sigma delta variety, increase the sampling rate to net several benefits such as reducing or ’shaping’ quantization noise or relaxing the design requirements for the analog reconstruction filter.

Here is a great tutorial on oversampling / upsampling and interpolation from Analog Devices:

https://www.analog.com/media/ru/training-seminars/tutorials/MT-017.pdf

dspGuru also has some great information on interpolation:

https://dspguru.com/dsp/faqs/multirate/interpolation/

@abraxalito

Thanks for that clarification - good to know. It's a linear interpolation, so very different from the interpolating filters used in the DACs themselves. The reference I found is at this link - https://www.pearl-hifi.com/06_Lit_Archive/02_PEARL_Arch/Vol_16/Sec_53/Philips_Tech_Review/PTechRevie...

Of course, all this only applies to compact disc digital audio. In case anyone is interested, I found an overview of the error correction approaches in other disc formats - https://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.462.3524&rep=rep1&type=pdf