This plug-in does something like that: Envelope follower / ducker
though I’ve no idea how silence can become “maximum volume”. What does that even mean? You want to replace silence with some kind of noise or some other sound? 
If the vocals are mixed in with other sounds, it’s probably not possible to separate them out (in high quality) without very expensive, purpose made software, (such as “Melodyne studio” ~ $700) and a lot of time tweaking it.