I maxed out my Buccaneer (who has Dead Mike) before they updated the Motherlode Mine Quest and now I'll never hear what Dead Mike says for the part, unless I create another pirate with Dead Mike and get him there, but its kinda stupid to waste time on a player only to do one thing.
Don't worry - when I was in Beta, dungeon voiceovers were added long after I passed them, and I know that this will work for totally skipped dialogue, too:
Find a friend! You can still see that dialogue if Motherlode is instanced (it is) and you're in it. Just quest through and you'll see the dialogue.
Just go back! If you can't find a friend, then Motherlode shouldn't be a problem for you.
Look around! Guides and walkthroughs are going up everywhere, and I'm sure that someone has a Motherlode runthrough on a Buccaneer.