Configuring default alsa device (hw:0,0) in asoundrc

I solved this issue as follows:

First, load the snd-aloop module:

sudo modprobe snd-aloop

This will create a new device called Loopback:

± % cat /proc/asound/cards                                                                                                                             !10017

 0 [PCH            ]: HDA-Intel - HDA Intel PCH
                      HDA Intel PCH at 0xc1814000 irq 60
 1 [HDMI           ]: HDA-Intel - HDA Intel HDMI
                      HDA Intel HDMI at 0xc1810000 irq 61
 2 [Loopback       ]: Loopback - Loopback
                      Loopback 1

From the information shown above we have to create two hw devices:

1. hw:0,0 (PCH is my main sound card).
2. hw:2,1 (The Loopback virtual device).

Normally, hw devices have the form: hw:X,Y. For our purposes, it seems that Y for the main card is always 0 while for the Loopback we have Y equals 1.

Now, in order to make all of this work, we need a ~/.asoundrc file with the following:

pcm.!default {
  type asym
  playback.pcm "LoopAndReal"
  #capture.pcm "looprec"
  capture.pcm "hw:X1,Y1"
}

pcm.looprec {
    type hw
    card "Loopback"
    device 1
    subdevice 0
}


pcm.LoopAndReal {
  type plug
  slave.pcm mdev
  route_policy "duplicate"
}


pcm.mdev {
  type multi
  slaves.a.pcm pcm.MixReale
  slaves.a.channels 2
  slaves.b.pcm pcm.MixLoopback
  slaves.b.channels 2
  bindings.0.slave a
  bindings.0.channel 0
  bindings.1.slave a
  bindings.1.channel 1
  bindings.2.slave b
  bindings.2.channel 0
  bindings.3.slave b
  bindings.3.channel 1
}


pcm.MixReale {
  type dmix
  ipc_key 1024
  slave {
    pcm "hw:X1,Y1"
    rate 48000
    #rate 44100
    periods 128
    period_time 0
    period_size 1024 # must be power of 2
    buffer_size 8192
  }
}

pcm.MixLoopback {
  type dmix
  ipc_key 1025
  slave {
    pcm "hw:Loopback,0,0"
    rate 48000
    #rate 44100
    periods 128
    period_time 0
    period_size 1024 # must be power of 2
    buffer_size 8192
  }
}

Modify hw:X1,Y1 with the values for your main card (in my case hw:0,0).

You can test that this configuration works by playing something in your computer. If you hear it, so it is fine. Now let's play something in the computer, and record it from this virtual device with ffmpeg:

ffmpeg -f alsa -i hw:X2,Y2 -c:a libmp3lame -b:a 256k -vn capture.mp3

Where hw:X2,Y2 has to be replaced by the Loopback device, in this case hw:2,1.

I suppose that using arecord should work too.

Backup your ~/.asoundrc - if you have one - and add the following, to a new ~/.asoundrc:

pcm.!default { 
 type plug 
 slave { 
 pcm "hw:0,0" 
 } 
}