WebAudio Worklets not working in Firefox

[Andonvr]

Please include the following in your bug report:

Version of emscripten/emsdk:
Anything from 4.0.8 to 4.0.13

Repo for reproducing is right here <--

This works in Chrome and Safari, but is broken in Firefox.

I followed the guide on Wasm Audio Worklets from here, and adjusted it slightly.
Basically, I added a class to hold a gain value, which I expose using Embind. When creating the Audio Worklet, I pass an instance of it. Later, I change the value of this, and the gain adjusts.

I compile, using CMake, and run using the most simple Vite project.

All of this works perfectly in Chrome.
But in Firefox, when creating the Wasm Audio Worklet (and not anywhere before that), it throws the error RuntimeError: index out of bounds.

With the compiler flag WEBAUDIO_DEBUG, it even prints "Registered new WebAudio object [object AudioWorkletNode] with ID 2" before failing.

[cwoffenden]

There are a collection of web audio tests here:

https://github.com/emscripten-core/emscripten/tree/main/test/webaudio

These build with nothing more than:

cd $EMSCRIPTEN/test/webaudio
emcc -sAUDIO_WORKLET -sWASM_WORKERS -o index.html audioworklet_params_mixing.c
emrun index.html 

These are complex examples with multiple nodes being mixed with parameters, tested on all browsers. If you can get your repro case down to a few one-liners and not requiring dependencies (Node, etc.) I can take a look.

I'm interested in any fail cases.

[Andonvr]

Fair, sorry!

I just pushed a new commit to the repo, reducing it to the barest minimum. See build command in the README.md.

[cwoffenden]

I just pushed a new commit to the repo

Perfect, thanks. I'll take a look tomorrow.

[Andonvr]

Thank you!! 🙏

[Andonvr]

I discovered something new: In my reproducible example, the user clicks a button, which instantiates the audiocontext and creates the Audio Node. Which fails in Firefox.
But if the context and the Audio Node is created immediately on page load, the bug does not happen.

In this branch everything seems to work.

(Note that I needed the WasmModule to be instantiated first, so I changed the build to be a JS-Module output, and added a custom .html to instantiate it myself. As far as I can tell, this is not what caused it to work.)

[cwoffenden]

I didn’t get time to look today, I will try tomorrow.

[cwoffenden]

I took a look and it works for me. I built your example (with the shell file) and placed it here along with one of the existing Emscripten tests:

https://wip.numfum.com/cw/2025-08-20/index.html

I ran this on macOS Sequoia and on Windows 10 (I can test on 11 when I trek the office later). Firefox works just fine (as does Chrome and Safari).

In the Emscripten example it prints the steps to console so you can see the init process.

From what you describe it might be a timing issue of the wasm module not being ready (if you look in the examples, I slowly go through the steps of setting each stage up).

[cwoffenden]

Hey, @Andonvr. Did you replicate this further, or can this be closed?

[Andonvr]

Sorry, yeah! Wasn't available to yesterday, and forgot today.
Your examples definitely both run for me! Curious.

"might be a timing issue of the wasm module not being ready"

This felt weird first, considering that in the first broken example I only set up the audiocontext and audionode when the user clicks a button. But: What if the onclick-function holds, like, a not-yet-instantiated "Module"? However then it shouldn't make a difference whether I run it locally, or your deployed version.

Well. Kind of out of ideas.
But since I found this workaround mentioned above, maybe that's alright.

[Andonvr]

Either way, thank you for your help 🙌

[cwoffenden]

I'll take a deeper look, but it'll be next week.

[sbc100]

Sorry, yeah! Wasn't available to yesterday, and forgot today. Your examples definitely both run for me! Curious.

"might be a timing issue of the wasm module not being ready"

This felt weird first, considering that in the first broken example I only set up the audiocontext and audionode when the user clicks a button. But: What if the onclick-function holds, like, a not-yet-instantiated "Module"? However then it shouldn't make a difference whether I run it locally, or your deployed version.

Well. Kind of out of ideas. But since I found this workaround mentioned above, maybe that's alright.

Could this be related to #24778 which just landed? Can you repro on tip-of-tree (emsdk install tot)?

[cwoffenden]

Could this be related to #24778 which just landed?

My two builds linked may have included that. I didn’t build with my audio branch but did have main since I’d merged.

[Andonvr]

Huh... I can't reproduce it anymore? It just works, across all branches on all browsers.. But it was definitely broken, I tried to solve this issue for a while before posting here, and I even had the error message seen in my original message. Damn.