Qsynth is a fluidsynth GUI front-end application written in C++ around the Qt framework using Qt Designer. Eventually it may evolve into a softsynth management application allowing the user to control and manage a variety of command line softsynth but for the moment it wraps the excellent FluidSynth. FluidSynth is a command line software synthesiser based on the Soundfont specification.
Kwave is a sound editor for the KDE environment. It is written with KDE/QT and is extendable through a powerful plugin interface. For the moment it supports .wav files and many other formats, recording/playback via PulseAudio, Qt Multimedia, OSS and ALSA and some simple effects.