AudioKit Synth One - Free & Open-source iPhone/iPad Synth 🎹
TwistedWave - TwistedWave is one of the unique audio editors that allow you to edit your music file online without any installation.
Sampulator - Make (and record) beats on your keyboard
WaveShop - Open source audio editor
Logic Pro X - Music production.
Audacity - Audacity is a free and open-source audio production software suite that includes a surprising array of editing tools and recording systems.