[ethyreal]

I guess it depends on your target market.

If it is for native english speakers then adding voice files for each kana sound would be a good thing.

For native Japanese speakers ( usually kids ) I'm not sure they need sounds.

I released something similar: http://ethyreal.com/projects/19-Kana-Trace

My app just focuses on writing the characters. My english responces mentions sounds and adding quiz functionality.

However even though I didn't think it would do well there I translated the UI into Japanese and most of my users come from Japan.

In Japan they just want their kids to practice and don't seem to care about sounds or quiz features.

But that's just my experience.