Virtual Audio Cable software allows you to transfer audio (wave) streams between applications and/or devices. It creates a set of virtual audio devices named "Virtual Cables", each of them consists of a pair of the waveform input/output devices. Any application can send audio stream to an output side of a cable, and any other application can receive this stream from an input side. All transfers are made digitally, providing NO sound quality loss (a bitperfect streaming). VAC behavior is similar to "What You Hear" (or "What U Hear", "Stereo Mix") feature of Sound Blaster Live! and Audigy cards. But it is only similar, not equivalent. If you simply need a function like "Stereo Mix" under Vista/Win7/Win8, there could be better to try to enable it in your audio adapter.
If more than one applications are sending audio to Virtual Cable device, VAC mixes all streams together. If more than one applications are receiving audio from Virtual Cable device, VAC distributes the same audio data among all targets.

VAC is useful to record application's audio output in real time (audio player, instant messenger or software synthesizer), or transfer a sound stream to another application processing it. You can, for example, use two or more software audio players/generators/synthesizers/sequencers to produce audio streams, sending them to Virtual Cablet device and record a mixed stream from the same Virtual Cable device, using any recording software - Windows Sound Recorder, Audacity, Sound Forge, WaveLab, Adobe Audition (formerly Cool Edit Pro), Gold Wave, Cakewalk/Sonar, Cubase/Nuendo etc.

With an ASIO wrapper like ASIO4ALL from Michael Tippach, you can use VAC in ASIO supporting applications.
If you use an audio encoder application that encodes a stream coming from a sound card, you can use VAC to supply such encoder with a stream produced by other application.
You can use VAC to capture an output sound stream from the application that doesn't allow to write it into WAV file directly. Unlike Total Recorder allowing you to simply save audio stream, VAC allows to route it in real time.
If you are using some Voice Over IP (VoIP) and/or Internet Telephony applications like Skype, you can use VAC to record your calls and conversations.

VAC needs no hardware audio card; it is a "virtual audio card" itself.
A detailed description is included into a free trial package.
Since VAC 4 is a WDM driver, there are some benefits but some (generally older) applications that use MME (waveIn/waveOut) interface can work better with the older VAC 3 version. This version also supports Windows 98/ME.
If you need to simply share your In/Out wave ports among several applications under Windows 2000 and earlier Windows versions, take a look to the Wave Clone software.

Features:
Local (console) session only (does not work via Remote Desktop or Terminal Services).
Windows 5.x, 6.x and 10.x platforms (32-bit and 64-bit).
Up to 256 virtual cable devices (some systems limit number of MME devices).
1..20 milliseconds per interrupt/event.
1..100 pin instances.
Almost any of fixed point PCM audio formats (1000..384000 samples per second, 8..32 bits per sample, 1..8 channels). Floating point formats are not supported.
Almost no sound latency with maximal interrupt/event frequency.
RTAudio support with notification events, clock and position registers. Clock registers are bound to Virtual Cables so all streams in each cable are coherent.
Unlimited number of Kernel Streaming clients connected to each port.
Signal mixing (with saturation) between output port clients.
PCM format conversion (sampling rate, bits per sample, number of channels).
Volume control features (both attenuation and boost).
Channel scattering/gathering mode.
Watermark control technique to improve stream stability with unstable applications.
Stream buffering technique to partially compensate bad application buffering algorithms.
Control Panel application to configure cables and watch their state.
Audio Repeater application that transfers from any recording to any playback device.

Whats New:
Fixed a bug caused Control Panel window to be truncated on the right on narrow displays.
Fixed minor bugs related to format range control.
Improved RT Audio (WaveRT) buffering mode, now supporting WASAPI exclusive mode event-driven streams with buffer duration down to 2 ms (two packets of 1 ms each).
Driver restart is no longer required to change maximum number of channels supported by the driver.
Added some explanations regarding real-time streaming, stream stability, ASIO etc., to the user manual.
Renamed format limiting mode "None" to "Driver range".

https://vac.muzychenko.net/en/