Spank?

There's two things at play here.

MP3 (or WAV, OGG, FLAC etc.) provide a way to encode polyphony and stereo and such into a sequence of bytes.

And then separately, there's Unicode (or ASCII) for encoding letters into bytes. These are just big tables which say e.g.:

• 01000001 = uppercase 'A'
• 01000010 = uppercase 'B'
• 01100001 = lowercase 'A'

So, what your text editor does, is that it looks at the sequence of bytes that MP3 encoded and then it just looks into its table and somewhat erronously interprets it as individual letters.

I think you are conflating a few different concepts here.

Can you comment on the specific makeup of a “rendered” audio file in plaintext, how is the computer representing every little noise bit of sound at any given point, the polyphony etc?
What are the conventions of such representation? How can a spectrogram tell pitches are where they are, how is the computer representing that?

This is a completely separate concern from how data can be represented as text, and will vary by audio format. The "simplest", PCM encoded audio like in a .wav file, doesn't really concern itself at all with polyphony and is just a quantised representation of the audio wave amplitude at any given instant in time. It samples that tens of thousands of times per second. Whether it's a single pure tone or a full symphony the density of what's stored is the same. Just an air-pressure-over-time graph, essentially.

Is it the same to view plaintext as analysing it with a hex-viewer?

"Plaintext" doesn't really have a fixed definition in this context. It can be the same as looking at it in a hex viewer, if your "plaintext" representation is hexadecimal encoding. Binary data, like in audio files, isn't plaintext, and opening it directly in a text editor is not expected to give you a useful result, or even a consistent result. Different editors might show you different "text" depending on what encoding they fall back on, or how they represent unprintable characters.

There are several methods of representing binary data as text, such as hexadecimal, base64, or uuencode, but none of these representations if saved as-is are the original file, strictly speaking.

Most binary-to-text encodings don’t attempt to make the text human-readable—they’re just intended to transmit the data over a text-only medium to a recipient who will decode it back to the original binary format.

Yes. Decoding a base64 encoded string will give you back the exact original data.

Importantly though, this isn't what you're seeing when you open files in a text editor as you describe in your original post, and if you copied the text of those files and saved a new copy it's very likely that it would not reproduce correctly.

yes, this method doesn't lose any bits. one of its primary use before was email which was strictly text only.

If you're on Linux, you can convert that to something more human readable by piping it to base64. It works with any file, but I'll use an image here:

cat image.webp | base64

Which yields:
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Copy that into a text file and pass it to base64 with the decode flag, and you'll get the original binary:

cat data.txt | base64 -d > data.bin

Inspect it to see what kind of file it is:

file data.bin -> data.bin: RIFF (little-endian) data, Web/P image

Rename it so you can just double-click it to open it:

mv data.bin data.webp

Enjoy the surprise.

You can also print files like that, scan them using OCR, and then restore them. A very inefficient way to do backups, but it works.

You can use a hex editor to view those files and even change them in some cases.

Something like this https://github.com/WerWolv/ImHex