Convert Stereo to balanced Mono

asound.conf

pcm.card0 {
  type hw
  card 0
}

ctl.card0 {
  type hw
  card 0
}

pcm.monocard {
  slave.pcm card0
  slave.channels 2
  type route
  ttable {
    # Copy both input channels to output channel 0
    0.0 0.5
    1.0 0.5
    # Copy and invert both input channels to output channel 1
    0.1 -0.5
    1.1 -0.5
  }
}

ctl.monocard {
  type hw
  card 0
}

pcm.!default monocard
ctl.!default monocard