I'm assuming this sound is for Goku, what you need to do is go to "C:\Program Files\Valve\Steam\SteamApps\email address\half-life\esf\sound\goku" folder.
Back up your old kamehame.wav sound, insert your new sound.
Things to know:
- Half-life uses 22050 HZ 16 bit mono, if you use anything else it will be converted everytime you create or join a game and you'll see the corresponding error.
- Open ended loops will crash the game in 22050 HZ.
Good luck, make sure it's an uncompressed .wav.