Ways to Pipe Sound Between SDR and Decoder

Note:- To avoid audio clicks or interruptions you may need to match the sample rates in the Audio Properties and software. For example when I was running HDSDR and "What U Hear" to WSPR-X I had to set HDSDR's output to 48,000 before it decoded.
You may also have to set virtual cables to the matching sample rate using the Windows controls, see here

Intermediate Sound Interfaces to Send Sound From SDR Programs to Another Program.

1)Stereo Mix 
2) and 4) will send audio back to the SDR program for transmit purposes. 5) Synchronous Audio Router for Windows 64 bit.

For passing audio in one direction from one program to another:- Many soundcards have a "Stereo Mix" , "What You Hear" , maybe some other name in their mixer settings. This is said to be often available but hidden.

Set the SDR program "RX Output (Speaker)" to the card with Stereo Mix. Set the digimode program to the input of that card and select Stereo Mix in the soundcard mixer. Note that soundcard SDRs must use a different card for the IQ input. 

A lengthy description and discussion about "Stereo Mix" here.
If you do not have the Volume icon look here, http://www.wintuts.com/Volume-Mixer-icon-on-taskbar later versions of Windows use "Settings" rather than Properties.
Note it is not always available. If you can install a second card then there is a good chance it will have this facility.
If a sound system has been automatically installed by Windows then using the manufacturer's installation disc may also install Stereo Mix. (Often along with much other unwanted enhancements.)

2) VB Cable
 http://vb-audio.pagesperso-orange.fr/Cable/index.htm One cable free, more by donation. One is all you need for receive.. I have not read too many reports about this, it has worked for me but some have suggested it does not always work.

You will have no speaker output. Set the SDR program "RX Output (Speaker)" to "VB Audio Point". Set the digimode input to "VB Audio Point".

3) Another possibility is to link two soundcards by  a real cable. I have not tried this. Although it is not usually recommended some have reported good results. A USB device may be used, preferably of reasonable quality.

You would have to Set the SDR program "RX Output (Speaker)" to one card. Set the digimode input to the other. Connect the Line Out (Speakers) of the first to the Line In of the second. I guess speakers may be connected to the second card.

4) Virtual Audio Cable

Virtual Audio Cable is used to send audio streams between the SDR program and another program  http://software.muzychenko.net/eng/vac.html This is not free (From 25USD) but unlike the "free" ones many cables are able to be set. There is a demonstration for initial testing but DO NOT USE THIS FOR TRANSMITTING. A voice message is superimposed on the audio stream.

Only one cable is needed for receive. Set the SDR program "RX Output (Speaker)" to Virtual Cable 1. Set the digimode input to Virtual Cable 1. The VAC Audio Repeater may be used to send audio to the speakers.

 NOTE some time ago when I was first setting up Virtual Cable it took several seconds, the screen blanked, I thought there were serious problems! However, no damage was done! 
Select one cable, Click "Set" to install. (Or

more cables if you have a specific requirement) The default settings above have always worked with me but you may find it necessary to increase the Sample Rate (SR) range for applications that require higher, or lower, rates.

I have found this program able to be set and forgotten.
VAC appears as a soundcard in Windows and the SDR program's sound menu.

In the Windows Start Menu is the "VAC Audio Repeater" which enables the VAC stream to be passed to a speaker output.

5) Synchronous Audio Router for Windows 64 bit.

http://sar.audio/  Free, is somewhat more complex to setup.

"Donationware" Voicemeeter is said to be able to stream audio from one source to two others so the received audio can be heard. http://vb-audio.pagesperso-orange.fr/Voicemeeter/index.htm

Stereo Mix is not always visible, sometimes not available. It is limited to one channel RX only. (Possibly two Stereo Mixes with two sound systems?)
VB cable offers 2 cables (2 ways should you use a SDR for transmit.)
The paid VAC offers a large number of 2 way cables.