Sound decoder questions

Hello,

I was looking on the internet for sounds for British locomotives, now I've finded a little movie from a sound decoder in a Bachmann Freightliner Class 66, sound great! But do you need a nother decoder to drive digital? Or is a sound decoder the same as a digital decoder, only with sound?
And is it possible to drive with a digital sound train on a anolouges system?
And how much volt das the locomotive need to drive on a digital system, cause I've got a roco LokMause, and it sent 15 Volt to the tracks. (Yes, it works with numbers )

Big chance I'm buying a Bachmann Class 66 with sound.

Thanks!

Kind regarts,

Ryan