Hi Product Hunt, I'm Hillel.

A few years ago, I built mudeo, an app for stacking video harmonies. Keeping the parts in sync was a constant struggle—every phone adds its own delay, and processing video just made it worse. Most users told me they really only cared about the sound anyway. So I started over. This time, I built an audio-only tool designed not just to record, but to actually help you learn.

Two things I prioritized from day one:

* It runs in your browser. No app to install, no account required to start. Open the link on your phone or computer and sing. Everything is timed to the browser's native audio clock, so the parts line up exactly.
* It's open source. The code is on GitHub (https://github.com/stackapella/stackapella), and the songs and courses are CC BY-SA. You can run your own copy; Stackapella Plus is simply how the hosted version sustains itself.

How it works:

* Learn your part: A guide voice sings with you, and notes light up as you hit them on pitch.

* Practice: The other parts carry on while you sing yours solo. You can slow the tempo down or loop tricky sections until you nail them.

* Stack the track: Record every part yourself. A quick calibration checks your device's delay first, ensuring your recordings line up perfectly in time.

* Share: Export your final stack as a link or video.

What else is inside:

* A free course that takes you from singing a single comfortable note to a full four-part harmony of Amazing Grace.

* A composition tool: hum a tune, and the app transcribes the notes and suggests harmonies.

* Two UI modes: an "Arcade" game mode (stars, combos, and boss rounds) or a "Calm" practice mode with zero distractions.

Pricing:

* Free: The app, the first seven course units, the free song library, and three custom songs of your own—with no account needed to start.

* Plus ($3.33/mo, billed yearly): The full library and advanced courses (barbershop, counterpoint, vibrato, yodeling, and more), featuring a 7-day free trial.

Native iOS and Android apps are on the way, but you can jump in on the web today. I’d love your honest feedback—especially regarding anything confusing in the first few minutes, and how well the timing holds up on your specific phone and headphones. I'll be here answering questions all day!