Since spessasynth_lib is a npm package, you need to do two things to make it work with browsers:

1. Bundle the code
2. Copy the processor

Well, that's straightforward. Copy the worklet_processor.min.js from spessasynth_lib/synthetizer/worklet_processor.min.js to the destination where the browsers can see it. Make sure that the path set in audioWorklet.addModule() works correctly in the minified file!

For example, you can make a basic script for building the project:

build.sh

# copy the worklet
cp node_modules/spessasynth_lib/synthetizer/worklet_processor.min.js --outfile=public/worklet_processor.min.js

esbuild --bundle --minify --sourcemap=linked --platform=browser index.js --outfile=index.min.js

then in your package.json:

{
  "scripts": {
      "build": "./build.sh"
    }
}

which allows you to npm run build to compile the project.

This is just an example, of course, make sure that your path is correct.

For that, you will need a bundler like webpack or esbuild. For simplicity, I recommend the latter.

npm install esbuild -D

Example file (named main.js in this example)

import {Synthetizer} from "spessasynth_lib";

console.log("yay, we have imported", Synthetizer);

esbuild main.js --bundle --minify --sourcemap=linked --format=esm --platform=browser --outfile=main.min.js

This will produce an output file called main.min.js and main.min.js.map for debugging. Make sure to exclude the latter from production builds!

Link the minified file to your HTML script.

<script src='main.min.js' type='module'></script>

That's it! It will work after that. Make sure to run the esbuild command every time you make changes.