Okay, I'll admit it. I'm a pretty big tabletop roleplaying geek. I generally play Dungeons & Dragons or GURPS twice a week in the evenings or afternoons with my friends. I'm currently the DM of a game loosely based on Castlevania (which is definitely one of my top favorite video game series). I like roleplaying (and board) games as a different kind of entertainment, apart from video games, movies, and television; if you're the game master, you have control over the way the story goes, and you get to play with as many friends as you want.
That being said, there are quite a few difficulties faced by a hearing impaired person who wants to play a roleplaying game. For one thing, there's no chance of subtitles - conversations can be fast and the action hectic, making it difficult to "rewind" and try to figure out what you missed. Depending on the group, if you miss one joke or bit of information, you may well be lost for the next half-hour or so. I know how that goes! As one of a half-dozen people, more or less, you can sometimes feel lost in the crowd, trying to ensure you are following along and able to participate. So, what can you do to ensure you have the best experience possible?
Explain your situation
Absolutely, don't allow things to go more than a few sessions before you reveal your hearing loss, unless you're totally comfortable. Remember that other people more than likely just want to help - if they don't, they're jerks! Your fellow gamers can ensure they address you clearly and to your front, and also keep an eye out if you look lost. A few times in my sessions I've had people ask if I've caught something, or if I need something repeated. Others keeping an "ear" out for you can be a huge help.
See if you can do without a screen or laptops
A dungeon/game master screen (example here) is sometimes used by the game master to block the players' view of his books, notes, die rolls and other secret information. Unfortunately it can also block something very important to you: their mouth. If this proves to be difficult for you, ask if they can raise their head when they speak, or simply do without a screen. Most people I know don't worry about one.
Laptops are also another interesting challenge to consider. Not only can they block mouths just as well, but sometimes laptop users will remain intent on their screen, not raising their head or taking their attention away from the machine to answer. (I'm guilty of this too.) Again, if it proves to be a problem, the best thing to do is self-advocate. It's either that or get a headache from straining to hear them.
Consider your location
If you're having a lot of trouble hearing the game master or the in-game conversation, consider where you are playing. The two game stores I typically play at are loud, with several competing games going on at once. Shouting, clapping, singing, music, video games and even normal conversation can make it difficult to separate the environment from your group. I've found that playing in peoples' homes is the best route, since you won't be able to shut up the people playing around you anytime soon.
But if a particular group is being especially loud or rowdy, you can talk to them, or to the staff at the store. The staff at a game store are interested in having repeat customers, and they know people won't come back or stay long if they're irritated by another group. Chances are other people around you are annoyed at the same thing.
Pick your seat carefully
The round-table type of seating seems natural for most groups. That's nice because it makes it easy to see everyone's face, if they're not buried in a rulebook. But even in that kind of situation your personal seating choice can be important. Sitting next to the game master or the talkative person in the group can ensure that you won't miss much. I prefer sitting near the game master because then I can quickly nudge him or her for a verification of what he or she just said. You can also choose to sit away from a radio or the television so you can concentrate on the conversation.
Notes - in the game and after
Sometimes, if a lot is going on, and the group is joking around or engrossed in a difficult or high-tension game, it can be easier to use notes instead of adding your voice to the fray. Scribble your question on paper, or type it on your laptop screen or cellphone and pass it to the person you want to ask. This is also a neat way to communicate just between characters if the plot calls for it. You can prank your fellow players and make sure you and your buddy are on the same page.
If someone is taking notes on the plot as the game progresses, you can ask to peruse their notes after the session and see what you missed, if anything. Or take notes on the session yourself - have someone else who was there look them over and see what was missed. That would be a good way to see what you are consistently missing (jokes that lead to plot branches in the middle of a loud battle, etc.) and let others see how to help you.
Read the book
This isn't something that needs to be said to most RPG players, but reading the book for reference is invaluable. Sure, you shouldn't look up a mysterious creature in the middle of an encounter, but if the game master describes a setting or another relevant piece of info from the books, you can look them up later to make sure you followed. If you suspect something is coming up in a future session from the books, you could also reread that information before the game. There's the chance of being spoiled for the particular game you're playing, of course, but with a good story and your interest in the game little would be lost.