Sonic Pi is described as 'Soinc Pi is a live-coding audio software. Learn to code creatively by composing, improvising or performing in an incredible range of styles from classical through to algorave' and is a music production app in the audio & music category. There are more than 10 alternatives to Sonic Pi for a variety of platforms, including Windows, Online / Web-based, Linux, Mac and Self-Hosted solutions. The best alternative is SuperCollider, which is both free and Open Source. Other great apps like Sonic Pi are Csound, Synthesine, Faust and Nyquist.
Synthesine is a 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.
Faust (Functional Audio Stream) is a functional programming language for sound synthesis and audio processing with a strong focus on the design of synthesizers, musical instruments, audio effects, etc.
Nyquist is a language and IDE for sound synthesis and music composition. It is implemented in C and C++ and runs on Win32, OSX, and Linux. Nyquist combines a powerful functional programming style with efficient signal-processing primitives.