"aperture_alpha.glados.01_01_helloAndWelcome"
{
   "channel"      "CHAN_VOICE"
   "volume"      "VOL_NORM"
   "pitch"         "100"
   "soundlevel"      "SNDLVL_NONE"
   "wave"      "aperture_alpha/glados/glados_01_01.wav"
   "soundentry_version" "2"
   "operator_stacks"
   {
      "update_stack"
      {
         "import_stack" "update_dialog"
         "mixer"
         {
            "mixgroup" "announcerVO"
         }
      }
   }
}

And the sound plays just fine. The problem is just that it doesn't lower the volume of the background noise.
It doesn't surpress background music played as raw from an amibient_generic.
It doesn't surpress background music played as soundscript from an amibient_generic.
It doesn't surpress the standard industrial soundscape, which plays in the background of every Hammer map without a custom one.

Anyone knows what I have to do?
Also, even if it did surpress the background noise, my sound would still be too quiet. Can this be fixed by increasing "volume" "VOL_NORM"?

come on, nobody knows that? What about the guys that already implemented custom voice files? Skotty? josepezdj? Has someone ever gotten it to work that their sounds silence the surroundings?

Original VALVE sound

"announcer.OpeningExercise01"
{
   "channel"   "CHAN_VOICE"
   "volume"   "VOL_NORM"
   "pitch"   "100"
   "soundlevel"   "SNDLVL_NONE"
   "wave"   "vo/announcer/OpeningExercise01.wav"
   "soundentry_version" "2"
   "operator_stacks"
   {
      "update_stack"
      {
         "import_stack" "update_dialog"
         "mixer"
         {
            "mixgroup" "announcerVO"
         }
      }
   }
}

COPIED version:

"apertureAlpha.announcerTest"
{
   "channel"      "CHAN_VOICE"
   "volume"      "VOL_NORM"
   "pitch"         "100"
   "soundlevel"      "SNDLVL_NONE"
   "wave"      "aperture_alpha/glados/openingexercise01.wav"
   "soundentry_version" "2"
   "operator_stacks"
   {
      "update_stack"
      {
         "import_stack" "update_dialog"
         "mixer"
         {
            "mixgroup" "announcerVO"
         }
      }
   }
}

Then, I implemented this copied version.

What drove me nuts: When I played both sounds next to each other, the official sound would silence the surroundings, my custom one not. I thought, deep inside the game engine, the official soundscript had been changed.

Today, I finally found it: The "mixgroup" function can only to be applied to the person it referends to. announcerVO works only for announcer. There doesn't need to be a generic_actor-entity inside the map, BUT the speaker must be the first word of the soundscript's name before the dot.

Therefore THIS works perfectly:

"announcer.ApertureAlphaAnnouncerTest"
{
   "channel"   "CHAN_VOICE"
   "volume"   "VOL_NORM"
   "pitch"   "100"
   "soundlevel"   "SNDLVL_NONE"
   "wave"   "vo/announcer/OpeningExercise04.wav"
   "soundentry_version" "2"
   "operator_stacks"
   {
      "update_stack"
      {
         "import_stack" "update_dialog"
         "mixer"
         {
            "mixgroup" "announcerVO"
         }
      }
   }

I hope this will somewhen help somebody.

BUT I've finally found the real error. And it works now. I've heard my own sounds in action. And all you need to do:

Place your sound in:
vo/<your actor>/sound

e.g.
vo/glados/aperture_alpha_glados_01.wav

and make sure to use the correspondent mixer, in my case "GladosVO".

If the sound is not in the vo folder, the mixer won't work. I can't understand why, but it finally works.