I have exactly the same problem. I hear all the sounds and music but from the start there are no voices. I clicked all the boxes too... Maybe it's a problem with the version of the game...
Actually the whole game is voiced, but in Japanese, the characters take less space than in English, so some of the stuff which were said by charas within 1 dialogue are split into 2 boxes for English, making the second part having no voice~
I actually found it really weird that people were saying that, since I didn't have any problems with mine, but then realised I could understand Japanese, so I didn't pay much attention to the fact that it was split in 2~
Not necessarily. This can happen with DMMd as well. Nitro+ games typically use OGG files for Music/Sounds/Voices. The only difference is that the Voice's are using 'Monaural' (Mono) sound. This means that they will play sound on ONE channel. Stereo sound outputs on two channels, "Left" and "Right". Typically on a "Stereo" setup, a Mono channel will play through both the Left and the Right channels.
If your sound card is set up for Surround sound speakers (5.1 channels), but you don't have a surround sound set, the voices will not be 'audible'. They usually go through a "Center" channel, if you have a surround sound speaker set.
If you only have 2 Desktop speakers or headphones, they are designated as "Left" and "Right", so there is no Center channel.
I honestly don't know what Sound card you have, but you can usually find the options by Right-Clicking on the Speaker Icon in the bottom-right of the screen, then select "Playback devices" from the text boxes. From here, click on your speakers, then click the "Configure" button and make sure your Configuration is set to "Stereo". Follow the rest of the wizard.
I hope that might help you.
D'oh. XD Ah well, that is also a solution... Haha, have fun!