I appreciate the theming and contrast of voice vs. writing in this very much, especially how a letter brought them back together and how Nagi managed to overpower Michiru's voice in the end, signifying her growth. They both liked similar things about each other, they had the same ideal. It merely found a different outlet.
But I must ask whether these girls have ever heard of telephones before... You can call each other even if you are in different countries. And a letter enthusiast like Michiru should have been pining for a pen pal from Japan, surely? It's not a big criticism, I simply don't enjoy unnecessary separation to begin with and them making no effort to contact each other for 2 years seems rather forced. Sometimes logic is lost in the desire to underline a theme and a poignant moment like "burning my voice into your mind", which would lose it's power if they just called each other all the time.
Obviously it has a happy ending and ultimately two years are not that much in the grand scheme of things. All that maters is that they are together now.