Speaker Phat + Retropie Voume

For starters your /etc/asound.conf should look something like the below.

IE: You should have a softvol plugin somewhere in the chain, because the PCM hardware doesn’t directly support any kind of volume control.

With that in place, drop to Terminal (F4) and run alsamixer then crank the volume up as high as you need.

pi@retropie:~ $ cat /etc/asound.conf
pcm.real {
  type hw
  card 0
  device 0
}

pcm.dmixer {
  type dmix
  ipc_key 1024
  ipc_perm 0666
  slave.pcm "real"
  slave {
    period_time 0
    period_size 1024
    buffer_size 8192
    rate 44100
  }
  bindings {
    0 0
    1 1
  }
}

ctl.dmixer {
  type hw
  card 0
}

pcm.softvol {
  type softvol
  slave.pcm "dmixer"
  control {
    name "PCM" # Masquerade as the default "PCM" sound device on Pi (for EmulationStation)
    card 0
   }
}

pcm.!default {
   type plug
   slave.pcm "softvol"
}