When trying to start the sound ingame, the sound won't play, but the console shows the following error:

[Sound] S_StartSound(): Failed to load sound 'vo\aperture_ai\escape_02_miscbabble-22.wav'. File is missing from disk/repository.

That's strange - if the file is missing from disk, why could I use it in the editor?

Another error was:

[Sound] S_StartSound(): Failed to load sound '*#music\portal_still_alive.mp3'. Can't create mixer.