The best open source alternative to CheVolume is EarTrumpet, which is both free and Open Source. If that doesn't work for you, our users have ranked four alternatives to CheVolume, but unfortunately only two of them is open source.
Ear Trumpet - Volume Control for Windows Supported operating systems Windows 10 Credits David Golden (@GoldenTao) Rafael Rivera (@RiveraR) Ear Trumpet icon created by Artjom Kormanfrom from the Noun Project
Synchronous Audio Router is a software-based audio routing driver for Windows. You can create any number of Windows audio endpoints, either playback or recording, and route them using any digital audio workstation software which supports ASIO, all with low latency.