SoundVolume - sets the volume of the sound before it is played - affects every time that sound is played thereafter.
ChannelVolume - sets the volume of a playing sound and only that one instance of the sound.
The final volume that you'll hear the sound at is SoundVolume * ChannelVolume.
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Edit: I use SoundVolume to balance my sound effects as they're loaded, then to set the overall volume for my game I use a function called PlaySoundEffect which sets the ChannelVolume of the new sound to the game volume level straight after playing. For looping sound, I wrap them in a type, so that I can find their channel to change the ChannelVolume if the player changes the volume mid-game.
ChannelVolume - sets the volume of a playing sound and only that one instance of the sound.
The final volume that you'll hear the sound at is SoundVolume * ChannelVolume.
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Edit: I use SoundVolume to balance my sound effects as they're loaded, then to set the overall volume for my game I use a function called PlaySoundEffect which sets the ChannelVolume of the new sound to the game volume level straight after playing. For looping sound, I wrap them in a type, so that I can find their channel to change the ChannelVolume if the player changes the volume mid-game.