For a hopefully suitable Android app, I've checked with my app search by permission for a task app requesting the
RECORD_AUDIO permission, indicating it should have voice support. That yielded 10 apps, which I checked, and here are some at least partly matching your requirements:
This app offers a widget for voice-to-text to add a task. Unfortunately, this app has no sync capabilities at all. However, Google Calendar support is on the todo list (oops – on the one of the dev, of course), so this part might be a matter of time only: once added, your tasks are accessible cross-platform via your browser, and also from a bunch of other tools also supporting Google Calendar. At least worth to keep an eye on: last update was just published 2 weeks ago, so development seems to be active still. If you're sitting "on the other side", the dev also has a Klingon variant with the same feature set: Klingon ToDo Agenda ;)
Star Trek ToDo Agenda with voice-to-task widget (source: Google Play; click images for larger variants)
This is a pretty extensive ToDo app, with "Cloud Support" built-in. From the app description:
- Enjoy an automatic, 24/7 sync of all your tasks, across all your devices– even if you’re offline
- due dates, recurring dates, sub-tasks, task priorities, sub-projects and color-coded projects
- Integrate with Dropbox and Google Drive to add files to your tasks
- Add tasks from any app on your Android by sharing with the Todoist app
- Use Todoist on your Android Wear smartwatch to get things done
Todoist (source: Google Play; click images for larger variants)
Todoist supports voice input via Google Now, and supports 13 platforms (see second screenshot), including Android, iOS, Windows, and MacOS. With its support via Web and Gmail, you should be able to access your tasks from everywhere – so this app should even be a perfect match to your requirements.
The last candidate I want to mention is Any.do – on of the top-rated apps in this category. From this app's description:
- Access all of your lists at home, the office, or on the go. Your Any.do task manager stays perfectly synced on your mobile, web app, or Chrome
- Use voice entry to speak you tasks into a list, or type with our powerful auto-suggest feature
As you can see, this also fully matches your requirements. It syncs with Google Tasks, which should answer most of the "availibility on platform X* questions.
For other alternatives, feel free to use above mentioned app search by permission, or directly check with my list of ToDo apps.