Why is "how would our lives change?" not correct?
"Ons leven" is singular. The plural would be "onze levens".
Your suggestion would make a little bit more sense though, I'll see if it can be fixed.
In English lives would be grammatically correct.
Not necessarily, say a husband and a wife we discussing a major change and asking how it would change their collective life together.
Without any context one assumes no "collective" life is meant, unless we are Borg, of course.
The context is right there, in the unequivocally singular pronoun "ons" (and the singular "leven"...).
Is there a difference in sound between "zou" and "zal"?
Yes, zou sounds a little like 'cow' but with a z (zow), and zal sounds more like 'all' with a z (zall).
"How will" would be "hoe zal"?