There's no such thing as 'should have been belted out' - just because they're done that way on stage (often necessarily, as Sam Barks has said in a few interviews, to reach the back of the theatre) doesn't mean that's the 'correct' way to do it, or that there even is a correct way. A lot more subtlety is possible on camera and it would have made no sense at all for the characters to just stand there belting.
The movie is not a carbon copy of the stage show, anyone watching it thinking it is will inevitably be disappointed. Take it isolation for what it is, and it's a different matter entirely.
OK, well I didn't think I needed to, but I will add that that, quite clearly, is my opinion.
To me, the emotion of the songs was lost due to the decision to make them speak-sung. Not saying people can't enjoy or that it's not the 'correct' way to do it, just that I thought it made the songs less powerful and, as a result, inferior to other versions.