Custom Sounds in map - how to upload them in BSP? [SOLVED]
maybe try the snd_rebuildaudiocache command in game?
One tip: in order to have a clearer view into pakrat you can choose to see the packed elements list "As a Tree" by going to View menu 
I actually didn't do anything with the paths.
I'm not sure what I'm suppposed to do. I just chekked whether the "Path fixup on add file" was on "Ask", and it was.
What exactly am I supposed to do?
On my friends computer, or on mine?
What will it do? Create a .cache file that I have to put in the .BSP?
EDIT: I tried doing what you said, then I recompiled my map, and added the sound files to the .BSP. still nothing.
EDIT2: OK OK OK!
My friend just noticed that in his console it says that the sound "Can't create mixer".
I've been experiencing this before, but somehow solved it. Does this help solving the problem?
If you're using .Wav, then make sure it's 16 bit. Put it into audacity and export it again. It should be the default option.
Musical website Moddb
If you look in the picture above, they're all mp3 
Are you using a custom file path for your sound files? Pakrat tends to use the default file path for resources since it makes it easier for other people to use your map that way.
My soundfiles are in SteamSteamappsCommonPortal2Portal2soundTemp
Is that custom?
Where is it supposed to point to?
This is how it all looks. Do you need more info?
http://oi60.tinypic.com/30a4z6t.jpg
http://oi60.tinypic.com/30a4z6t.jpg
If you have your settings like that, then, after you browse your music files and click on the 'Add' button, it should have asked you to fix up the paths for your files; and you must have said "Yes to all". Don't you remember that step? 
After you have correctly added your sound files into your BSP, in the case that you (or your friend) can't hear them in game, you should always check what your console says:
1. If you have the error message "Can't create mixer" = Check out that you DID create your sound file at 44100Hz (no matter if it's .MP3 or .WAV). Then check out your ambient_generic and recreate it to see if that fixes it.
2. If you have the "missing sound from disk/repository" = That can be fixed quickly by rebuilding or updating your sound cache entering one of these commands into your console:
- snd_updateaudiocache
- snd_rebuildaudiocache
However, once you have your audio files into the BSP the game looks into it and you should not have that latter error message...
Hmmm..... can you say what audio programs are you using? sometimes there are some of them that automatically change the sample rating when you are about to save them, because there are some settings to do so in your program's 'Preferences'
useful tools and stuff here on TWP 
[spoiler]
[/spoiler]I am using Audacity.
I've never been fiddling with what kind of sound file it is, so it IS 44100Hz. Otherwise, I wouldn't be able to hear the files myself would I?
He was getting the error: "Can't create mixer". I got this too, once. Not that my sound files were wrong at that point. They were exactly the same.
EDIT: Now when I think about it, it was my friend who recorded the sound files. Then he send them to me, as .wav. Then I editted them and exported them as .mp3. Can this have influence?
EDIT2: New finding. As I watched some videos on youtube about this, one guy just opened pakrat and pressed "Auto". Then it found all his soundfiles... This does not happen to me.
The only thing pakrat finds is that .txt file that I don't need
Man, do you mind to post or pm me your BSP + sound files so I can check them, try to pack them and test them in game?
useful tools and stuff here on TWP [spoiler]
[/spoiler]No, that shouldn't be a problem.
These are just the first 8 voice files which are used in the first map.
And then the first map's .BPS of course.