I know someone can help me out on AB! When it sounds wrong, we say it's this. I've got ?npronounceable. What's the first letter? I can't see why it would be a 'u' as that would mean we couldn't pronounce it, nor can I see how it could be a 'i' Please help!
Neither Concise OED nor Chambers has inpronounceable but both list unpronounceable. Beyond that most ( but not all ) words beginning "p" have the prefix "im-" rather than "in-" to create the opposite.