This sounds like a farily persistent behavior. Have you talked to him about it? That would be the first step, is to politely discuss what type of mood you're trying to set, and ask him to tone the humor down.

well yes frankly, I have. the problem is, not so much that he's a jerk or something, but that he just has a lot of difficulty keeping focused. He's a good friend, and I would very much like o avoid sacking him if need be, i was just wondering if anyone had any tricks of switching from story mode to dialogue and little conversation things that can help keep a group on track.

Ah! Easily distracted player. That changes things immensely.

It's been some time since I've RPed face-to-face with a group, but I've always been fond of having both visual and vocal cues to make it clear when it's "game on".

• When narrating (gamemastering), I speak a bit more slowly and deliberately, and even lower my tone of voice a note or two. (When speaking as an NPC, I tend to use a more appropriate voice and pace for them, but that's an advanced trick.)
• If I've got an actual screen, the screen goes up when it's game time, and goes down (covering notes) when it's not.
• I sit forward in my seat, often hunched over the table, when I'm narrating; if I'm just chatting, I'm kicking back in the chair.

I also enforce the rule of getting rid of distractions. No TV, no videogames; books are okay, as they won't distract anyone else, but woe betide you if your scene comes up and you're not ready! Sometimes I'll have thematic music to go with a scene, which can help to focus people's moods.

In this case, my suggestion below of an "Always Speaking in Character" rule might actually be a good one, if you think it'll help him focus.

Sack him. (Yes, the pun is intended, but it's still practical advice. :P)

I wouldn't be so quick to dump a player. However, if after talking about and he continues with the same behavior, then you may want to ask him to find another group.

Actually, I meant beat the crap out of him and put him in a sack.

I LIKE this idea. in general

Most of what I'd suggest has already been, so I'll just tack on a few thoughts and questions.

How close are you to the player? Are you friends or simply gaming acquaintances? Can you talk with him outside of game days? Is he friends with someone else in the group? Is everyone else upset with his wisecracking, or are there some people who don't see a problem with it? Objectively, are you the only person bothered by it?

I ask these questions because confrontations are never pretty. (When they are, it's usually because someone wasn't paying attention.) I have a suspicion that no one's talked to him previously because this player may - or will - react badly if called on his behaviour, and no one wants that. Determine in advance if there's any way to end the issue bloodlessly; if not, prepare the medical supplies (figuratively speaking).

For a much more flip suggestion, you could always implement the "Always Speaking in Character" rule. Since your game setting is a serious one, his joking would then be perceived as serious non-sequiters or the ravings of a madman, thereby punishing his character for his kidding around without making it appear arbitrary. (I do not, however, recommend doing this if you hope to keep the player.)

Easily distracted player? I think every group has one (or did), to some degree.
Mine? If she's too busy making dumb jokes to listen to important information I'm giving concerning...anything, then her character simply wasn't listening. She'll have to make due without all the facts, which could lead to her (PC's) death. The more characters she goes through, maybe she'll learn there's a time for dumbass jokes, and a time to get in character.
Of course if your player is playing a bard, then give him extra XP.

ah, but you see the character, while not a bard, is a bard substitute in many ways, being a more socially inclined character. its the out of game world references which really bother me, and the INTERRUPTING THE GM which is worse

have you tried the simple aproach of just saying "hey were getting off track here,theres a game going on" my group has allot of fun and we tend to get side tracked allot, a bit of that goes on. Im constantly just saying "anyways back to the game" and thats usually all it takes. most of the time people dont even notice there doing it,there just having fun

I find myself doing the "anyways, back to the game". It doesn't bother me too much, especially since some of the other players say it too.
Another good solution is base some of the night's experience on how well your players stay in character. I think he might figure out at some point why his 6th level character keeps getting attacked while his 13th level group keeps getting the kills.