Long story short it seems that StereoMix heavily used in past was a feature of the sound card and sound drivers, so... it's not possible to use it if it isn't present on the machine.
C:\Users\user\Desktop> ffmpeg -list_devices true -f dshow -i dummy
ffmpeg version N-94077-g01d8c72b95 Copyright (c) 2000-2019 the FFmpeg developers
built with gcc 9.1.1 (GCC) 20190621
configuration: --enable-gpl --enable-version3 --enable-sdl2 --enable-fontconfig --enable-gnutls --enable-iconv --enable-libass --enable-libdav1d --enable-libbluray --enable-libfreetype --enable-libmp3lame --enable-libopencore-amrnb --enable-libopencore-amrwb --enable-libopenjpeg --enable-libopus --enable-libshine --enable-libsnappy --enable-libsoxr --enable-libtheora --enable-libtwolame --enable-libvpx --enable-libwavpack --enable-libwebp --enable-libx264 --enable-libx265 --enable-libxml2 --enable-libzimg --enable-lzma --enable-zlib --enable-gmp --enable-libvidstab --enable-libvorbis --enable-libvo-amrwbenc --enable-libmysofa --enable-libspeex --enable-libxvid --enable-libaom --enable-libmfx --enable-amf --enable-ffnvcodec --enable-cuvid --enable-d3d11va --enable-nvenc --enable-nvdec --enable-dxva2 --enable-avisynth --enable-libopenmpt
libavutil 56. 29.100 / 56. 29.100
libavcodec 58. 53.100 / 58. 53.100
libavformat 58. 28.100 / 58. 28.100
libavdevice 58. 7.100 / 58. 7.100
libavfilter 7. 55.100 / 7. 55.100
libswscale 5. 4.101 / 5. 4.101
libswresample 3. 4.100 / 3. 4.100
libpostproc 55. 4.100 / 55. 4.100
[dshow @ 0000020a1ff08b00] DirectShow video devices (some may be both video and audio devices)
[dshow @ 0000020a1ff08b00] "HP Truevision HD"
[dshow @ 0000020a1ff08b00] Alternative name "@device_pnp_\\?\usb#vid_064e&pid_9301&mi_00#6&a1a3a63&0&0000#{65e8773d-8f56-11d0-a3b9-00a0c9223196}\global"
[dshow @ 0000020a1ff08b00] DirectShow audio devices
[dshow @ 0000020a1ff08b00] "Microphone (High Definition Audio Device)"
[dshow @ 0000020a1ff08b00] Alternative name "@device_cm_{33D9A762-90C8-11D0-BD43-00A0C911CE86}\wave_{412CABE0-42CF-45FB-96DF-D9A61252AADE}"
dummy: Immediate exit requested
Recently I updated my os from win7 to win10 and the StereoMix feature disappeared.
It turned out that windows has a new api called WASAPI and it works by allowing you to record the live audio from the computer by re-reouting the stereo output into the input. Audacity seems to be able to do it.
Can ffmpeg access WASAPI devices and allow them to be used for recording? So that I can bypass the problem using WASAPI like Audacity does.
