Go to where your game files are stored:
In your Steam Library, select Portal, select Manage (gear icon), then Manage > Browse local files

Open the "portal" sub-folder, then create a new folder named "custom".
Create another folder inside "custom" and name it whatever you want (example: my_mods).
Create three more nested folders named "sound", "ambient" and "music" so the final folder structure looks like this:

...\Steam\steamapps\common\Portal\portal\custom\my_mods\sound\ambient\music\

In this folder, place ANY sound file in .WAV format, then rename it to looping_radio_mix.wav
The WAV format should be PCM, 44.1 kHz, 16-bit. Either stereo or mono will work.
For help changing the format, search the Portal Guides section for "music".

(If you have the "Hide extensions for known file types" option enabled in your file browser, the file name should appear as "looping_radio_mix", with the "WAV Audio File" type.)

Restart the game, and radios will play your "custom" sound instead of the original music.
To make the custom sound "loop", add cue points to the .wav file, as described here:
https://developer.valvesoftware.com/wiki/Looping_a_Sound


A low-volume version of the original music (downloadable):
https://www.dropbox.com/s/4rn058lib72h12d/low_volume.wav

To completely silence the music, use a blank/empty text file.


To restore the original music: move, rename or delete your custom sound file.

(Use Option 3 instead of this!)

To make the game play like it did on the first playthrough, we need to make it think you have not yet received the Heartbreaker achievement for completing Portal.
Only the original few radios will appear in the game, with no red or green lights, the volume of the music will be normal, and the radios will not turn to face you when you pick them up.


First, set Steam to Offline mode.

Then exit Steam completely, either by right clicking the icon and choosing Exit, or by choosing Exit from the Steam file menu.

Then, delete(*) these two files:

...\Steam\SteamApps\common\Portal\portal\gamestate.txt
...\Steam\appcache\stats\UserGameStats_<numerical user ID>_400.bin

Start Steam. When prompted, choose START IN OFFLINE MODE.

Finally, play the game, with no extra radios.


To return to normal, set Steam to Online mode.


* If you are part-way through the Transmission Received achievement, and want to keep your progress, make a backup copy of the gamestate.txt file. When back in online mode, replace the new file created by the game with the backup copy.

As part of the March 2022 updates, Valve finally provided an option to disable the radios.

Using the developer console:
portal_disable_radios 1
(Or set it to 0 to enable the radios.)

Using the new "gamepad" user interface:
Run the game with the launch option
-gamepadui
then go to Options, select the Portals tab, then set "Post-game Radios" to Disabled.