sentence: competing technologies apple 's audio units avid 's avid audio extension digidesign 's real time audiosuite digidesign 's tdm ladspa , dssi for linux lv2 , a cross-platform , open source , liberally licensed audio plugin standard microsoft 's directx plugin mark of the unicorn 's motu audio system jack audio connection kit , an open source sound server allowing flexible audio routing between apps propellerhead 's rack extensions programming languages steinberg 's vst sdk is a set of c + + classes based around an underlying c api .