Rosegarden is free software digital audio workstation. It has MIDI and audio playback.
So I connected my Casio CT 670 keyboard, and played midi files via Rosegarden.
Keyboard has 4 channel so I also use software synth (qynth) for having more channels. But If you want to have good sound via software synth you have to have good sound card (asio drive prefered).
yum install qjackctl fluidsyth fluid-soundfont qsynth rosegarden4
qjackctl -> this is for connecting qynth your midi device and rosegarden
qsynth --> software synth, you can find soundfonts from internet