I think, athough I could be wrong:
Sound volume sets the volume for that sound so if you've got a loud sound that you want to balance with your other sounds you can set it's volume to .5 say. Then it's channel volume will be from 0 - 1 which in reality go from 0 - .5. Also you can use a sound twice, so if you have say a bang noise you might want to use the sound "Bang" at a high volume and a low volume therefore these would be channel volumes and not sound volumes.
Sound volume sets the volume for that sound so if you've got a loud sound that you want to balance with your other sounds you can set it's volume to .5 say. Then it's channel volume will be from 0 - 1 which in reality go from 0 - .5. Also you can use a sound twice, so if you have say a bang noise you might want to use the sound "Bang" at a high volume and a low volume therefore these would be channel volumes and not sound volumes.