Aquila

Aquila' is an open source and cross-platform DSP (Digital Signal Processing) library written in C++ that offers I/O functions, filtering, FFT, windowing functions, feature extraction and other functions suitable for both audio signal processing and general DSP work.

The library is essentially two parts, the basic DSP C++ class library itself which does compile and work on OS/2 and most parts of it do so little I/O that it borders on being system agnostic, it may however require a relatively recent C++ compiler to work as version 3 is coded to the C++ 11 standard and relies on CMake to compile, but that should not pose any problems.
 * Portability and OS/2 compatibility

The supplied example code and auxiliary programs however are written with the SMFL multimedia library and the Qt UI/Widgets library. While a reasonable Qt port exists for OS/2, the SMFL port on the other hand is outdated and you may not get any of the example programs to work unless you port them. Neither the examples or auxiliary programs are needed or in most cases even useful so this is not a showstopper at all.

Version
Note that the library was largely rewritten for release 3, some features present in the v2 branch of the library have not yet been ported to v3, including the much used FIR and IIR filters, so you might want to take a look at both version of the library to judge if it fulfils your need. The v2 branch may be more portable to older compilers as well.
 * Last known 3.x branch: Aquila-DSP
 * Last known 2.x branch: Aquila-DSP v2.5

Licence

 * Open source software released under the MIT Licence.
 * Author: Zbigniew Siciarz

Links

 * Aquila homepage
 * Aquila-DSP repository - On GitHub