If you want to further improve the sound quality, the Newsmy NU-3001 Android head unit has an awesome digital amplifier, and when combined with Rocket Player and music ripped from CD in .FLAC format, you get some great low end and the tweeters on the dash come alive.
Besides the door panels rattling, the i-MiEV sounds as good as my home system.
https://partsouq.com/en/catalog/genuine/unit?c=Mitsubishi&ssd=%24HQARW1d0AAIKDUg0JS5dYWRge2VwXV5VWA%24&vid=5099&cid=4&uid=124621
Part number 8720A147, front 15 watt speaker, $72.87 each. (same as linked above by misterbleepy), so same speakers for both EU and NA cars. Expensive, but these do sound good and have plenty of volume (I have the ES, which has these 15 watt speakers).
The radio in the car definitely makes a difference. The stock ES radio in the NA cars is pretty good. Replacing it with a single-DIN Pioneer radio sounded worse, and the NU-3001 head unit sounds better. These were all in the same car with the same speakers. It could be that the clones use cheaper radios, and that's why there are complaints about the iOn/C-Zero sound system.