Small renaming and bug fixing in SamplePlayer
Created by: akien-mga
As discussed on IRC, the first commit renames _voice_count to polyphony in SamplePlayer to be consistent with SamplePlayer2D. This also makes it consistent with SpatialSamplePlayer and SpatialSound_Server. On the other hand it will break code that rely on GDScript's set_voice_count().
The second commit fixes some copy-paste mistakes where linear/db volume functions had wrong argument hint or even wrong bound function.