PortAudio is described as 'cross platform, open-source, audio I/O library. It provides a very simple API for recording and/or playing sound using a simple callback function' and is an app in the Education & Reference category. There are nine alternatives to PortAudio for a variety of platforms, including Mac, Windows, Linux, Android and iPhone. The best alternative is JUCE. It's not free, so if you're looking for a free alternative, you could try Wwise or RtAudio. Other great apps like PortAudio are FMOD Ex (Free Personal), OpenAL Soft (Free, Open Source), KFR (Freemium, Open Source) and SoLoud (Free, Open Source).
RtAudio is a set of C++ classes that provide a common API for realtime audio input/output across Linux, Macintosh OS-X and Windows operating systems. RtAudio significantly simplifies the process of interacting with computer audio hardware.
OpenAL Soft is an LGPL-licensed, cross-platform, software implementation of the OpenAL 3D audio API. It's forked from the open-sourced Windows version available originally from the SVN repository at openal.org.
KFR is built for developers who need a powerful toolkit for audio and DSP applications. The framework is packed with ready-to-use C++ classes and functions for various DSP tasks from high-quality filtering to small helpers to improve development speed.
Photon Micro GUI is a modern C++ (esp. C++14 and C++17) UI library. Featuring resolution independence and HDPI support, with modularity as another focal point, the library is made for general purpose use while also covering everything needed for audio plugin development.