Are the voice and background tracks merged together when filming hollwood movies?
To answer only the first part, film sound is often split into three parts: music, effects, and dialog. This allows dialog to be replaced for dubbing. This isn't a hard-and-fast rule but it's the general way it's been done for a long time. 

This is not how the sound is released, though. For release these are combined into a theatrical mix of two or more (for surround) channels, so the original three-track mix is no longer in play.