Hello, here is a little tutorial on the donationware VoiceMeeter Banana software mixer for Windows. I wrote this tutorial in the french forum, but I thought this could be useful here.
In red, the inputs IN, in yellow, the outputs OUT.
The hardware inputs represent the audio hardware devices that are plugged to the PC. In this example, my USB mic is plugged in Hardware Input 1.
The virtual/software inputs are the programs running in the PC and emit sound. Here it could be REAPER or a Youtube video on the VOICEMEETER VAIO track.
(above "voix redirigée vers A1 et B1" = "input redirected to A1 and B1"
The outputs can be either hardware (here A1 is my internal sound card, A2 could be an external sound card), or virtual/software (B1 and B2).
For REAPER, choose WASAPI shared mode and the inputs/outputs as shown below:
In this configuration, B1 is routed to REAPER after being mixed via VoiceMeeter.
Note that I could have chosen Voicemeeter Aux for the Input Device. The B2 output of VM would then go to REAPER.
In the same fashion, had I chosen Voicemeeter Aux Input as the Output Device, the REAPER output would go to the virtual input IN Aux of VoiceMeeter.
The reason why you should choose the driver WASAPI is WASAPI shared mode isn't exclusive, contrarily to ASIO or WASAPI exclusive (polled) mode. Therefore, it's the best mode when one uses several programs that need concurrent access to the audio, like the output of a DAW or the web browser on Youtube in the Youtube / VoiceMeeter / REAPER chain. If only one program needs access to audio, one can choose ASIO or WASAPI exclusive mode. The drawback of Shared mode with respect to exclusive mode is higher latency and limitation to 16 bits/44KHz. Note that Windows 10 has a redesigned WASAPI core that has lower latency than Windows 8.
edit (vburel): Remark: REAPER can also be connected to Voicemeeter Virtual ASIO driver (supporting up to 4 client applications)... While Voicemeeter output A1 is expected to be connected to your ASIO device (this is the optimal configuration able to work with 256 or even 128 sample buffering).
Finally, the tape recorder can record on hard disk any of the 5 outputs, and even the 5 outputs at the same time. Note that a right click on EQ opens a parametric equalizer on each output, and that the outputs include a limiter.
PS: don't forget to choose VoiceMeeter as default hardware in the Windows mixer, else it's shorted by Windows. On my setup, VM Banana is launched at startup. Also the VoiceMeeter settings allow to choose the size of the driver buffers, which has a direct effect upon the latency displayed in REAPER.