Very long ago, I wrote an interpreter in PHP for an arbitrary syntax that was essentially a subset of PHP. It had loops, recursive functions, and performed calculations with BC math (basically using strings for mathematical calculations instead of primitives since it had fewer capacity limits. No practical reason for it though). It taught me a ton about tokenization and how precedence works in an interpreter

I also made a simple mod tracker in JavaScript. It didn't use the audio API. It worked by changing the frequency of header values in WAV files it had embedded within it to play different notes from a sample. Surprisingly it actually changed pitches properly. But the embedded nature made it unfeasible to keep or play more than two samples at a time. It was a good lesson in editing binary, file formats, and browser limitations