Virtual Machines (VMware Player/Workstation, VirtualBox, Hyper-V).

Audio editors/recorders (Audacity, Adobe Audition, GoldWave, WaveLab, Sound Forge). Software synthesizers and studio/DAW (Ableton, Acid, AudioMulch, Band-in-a-Box, Bidule, Cakewalk/Sonar, Cubase, FL Studio, GarageBand, Logic, MiniMoog, n-Track, Nuendo, Reaktor, REAPER, Reason, Samplitude, Traktor, Vegas). Speech synthesis/recognition (Google Text-to-Spech/Speech-to-Text, Dragon Naturally Speaking). Live mixing/broadcasting (vMix, Open Broadcaster/OBS Studio, WireCast, VidBlasterX. Internet Telephony/VoIP/SIP/voice messengers (Skype, Ventrilo, MicroSIP). Use VAC to connect various audio applications together: VAC can create up to 256 independent Virtual Cable, allowing you to route up to 256 independent sounds at the same time.

Thus, you can record and/or process audio output of any application by any other application, pass sounds between them, and much more. When an application plays a sound back, any other application(s) can record this sound directly. VAC connects audio applications together in real time.

Nuendo 4.3