How to turn off GLaDOS’s voice in Portal using the console

You need to think with Portals!

Look up in the direction you are heading to the fire; you can spawn a portal on the wall up there, one to your right, and teleport out of danger.

After that, keep trying to reach the party to get your promised cake!

Ha! I did it by cheating: finding an existing sound mod for Portal and examining it :D. Here's how, for posterity:

Apparently, the Source engine does not look for sounds in the directory. It looks for script files instead. Those script files define where to look for stuff.

So I did the following:

• Opened the main Portal GCF (that's portal content.gcf) file with GCFScape;
• Extracted GLaDOS's voice script file (that's scripts\npc_sounds_aperture_ai.txt) to the directory where the game engine will look at, which, according to the Valve wiki, is the one where GameInfo.txt is in. So STEAMAPPS\millenomi\portal\portal, so that I have STEAMAPPS\millenomi\portal\portal\scripts\npc_sounds_aperture_ai.txt

Now Portal will look relative to the scripts file for sounds! …not. Apparently, if the file has the same name it's still looked for using the same search path that led Source to pick up the original file instead. Sigh. So, instead, noticing the working mod had file names that were different from the ones in the archive, I changed the script and the filename to a name that wasn't used anywhere else in the game (vo/aperture_ai/00_part1_entry-1__MODIFIED.wav). And BAM!, it worked. :D