Voice work is not important. If you've got the engine set up such that it's an option, just leave blank placeholder stuff in there, you can always worry about it later.
Likewise, resolution shouldn't really be a major concern. The game simply isn't going to have the level of detail that requires anything past the already pretty decent resolution you were working with, and I'm guessing you've found there are also performance implications?

If anything, I'd say your time would probably be better spent seeing if you can't improve your background tiles. Some simple 2d background work could do wonders without adding any strain to the engine at all.
It would also be nice if something could be done about all those maths problem puzzles, but we're probably too far in to the project by now for that to be a realistic prospect for change.
As it is, remember to have fun and take it easy, there's no rush and no obligation.