setting volume of a sound

As far as I know, every time you have a sound effect it automatically creates another channel. When you do this:

channel_variable=EmitSound(sound_variable)

Channel_variable is just the variable that the channel handle is. By doing this:

channel_variable=EmitSound(other_sound_variable)

Channel_variable changes; the sound isn't now replacing your first sound's channel. All that Channel_variable is is just the way of using other sound commands with the channel. Just this:

EmitSound(yet_another_sound_variable)

Will still emit a sound with another channel.
As for your second question, I see no problem whatsoever with changing your ChannelVolume. So far I've not encountered any problems with this.