If it includes both going in and going out, by simple reasoning, it does not mean simply going in. The sheep also hear and follow when out.
This is actually what eternal life consists in. An ongoing interplay between Shepherd and sheep is necessary to abiding in Christ.
You are simply not getting what I'm saying. Jesus voice,
within the context of the "my sheep hear My voice", is about gathering those who belong to Him
so they can begin to follow Him in and out. It is not about His voice being used constantly while the sheep are following. In this instance His voice is the Gospel which brings those who belong to Him into the fold.
The problem is people keep using this verse to mean as we are following, we are hearing His voice but once we are have come to the Shepherd (saved) we now have a visual. Our visual is the written word. We don't sit around listening for voices, even figuratively, we have already come, now we fix our eyes on Jesus and follow and that requires learning the word.
You are correct, the constant interplay is necessary but the "sheep hear my voice" verse is not it. The verse cannot be used to justify how we follow the Shepherd because it is talking about what happens to bring us to the point of following not the actual following.