Ok, here's how it works;
In the HP Phaser property, say you have a sound called 'Federation Phaser'
In you sfx folder you have two sound files called MyPhaserSound1.wav and MyPhaserSound2.wav
So your soundpak file would contain this;
Foundation.SoundDef("sfx/Weapons/MyPhaserSound1.wav", "Federation Phaser Start", 1)
Foundation.SoundDef("sfx/Weapons/MyPhaserSound2.wav", "Federation Phaser Loop", 1)
Foundation.SoundDef("sfx/Weapons/MyPhaserSound2.wav", "Federation Phaser Loop", 1)
The bit in italics must exactly match the name in the HP Property for that sound.
This assumes that the Start sound is MyPhaserSound1.wav and the Loop sound is MyPhaserSound2.wav
Your basically mapping your wav's to the name of the phaser in the HP property.
Edit: Fixed the italics tags