Synthesine is described as 'web playground for synthesizing sound waves from scratch. You can use it to create sine waves, white noise, and other building blocks of sound, and then pass these through modular components to synthesize more complex sounds like bells and whistles' and is an app in the Audio & Music category. There are more than 10 alternatives to Synthesine for a variety of platforms, including Windows, Online / Web-based, Mac, Self-Hosted solutions and Linux. The best alternative is SuperCollider, which is both free and Open Source. Other great apps like Synthesine are Sonic Pi (Free, Open Source), ChucK (Free, Open Source), Nyquist (Free, Open Source) and Faust (Free, Open Source).
AudioMasher is a live, in-browser audio and music programming environment. The app is based on the Sporth audio programming language by Paul Batchelor, is open-source and can be used online or via self-hosting.
Gibber is a creative coding environment for audiovisual performance and composition. It contains features for audio synthesis and musical sequencing, 2d drawing, 3d scene construction and manipulation, and live-coding shaders.