[db48x]

For audio you might also consider U+2581—U+2588, LOWER ONE EIGHTH BLOCK through FULL BLOCK. And then if you really want to go all out there are sixels, but that’s basically just an image; you probably lose the easy ability to compare them. On the other hand they’re not available in every terminal.

[rbanffy]

These blocks are great if you need more vertical than horizontal resolution. There are also newer symbols in the Unicode 13 spec that are just the line without the fill.

[db48x]

Oooh, I didn’t know about those! I’m going to go look them up.

[rbanffy]

There are more details here: https://en.wikipedia.org/wiki/Symbols_for_Legacy_Computing

The same group wants to include a couple others, from different platforms, but the Unicode Script Ad Hoc Group is concerned the new batch may not be as meaningful as the first one.