Imagination can be a powerful thing. It's unlikely that riding through some mud would do anything except make the bike dirty. I avoid it as much as I can just so I don't end up looking at a big cleanup job in my apartment. But if you insist, clear your mind by having the bottom bracket and wheel bearings serviced. A simple lube job should be good enough to alleviate any remaining concerns you might have about it and it shouldn't cost very much.