Okay guys, back to the game
As a GM, I am just as guilty (if not more so) as the players of making stupid jokes, and I have no problem at all with the odd gag- they are very much a part of the role playing experience for lots of players (and one many of us would be loathe to sacrifice). But when they do disrupt the game it is up to you as GM to take command of the situation and get it back on track.
You might mention before the game that you have a lot to get through, and that you are going to try and keep the focus on the game. No need to point the finger at anyone, and definitely no need to make any threats about docking XP (I hate that sort of stuff)
When someone makes a joke, let everyone (including yourself) enjoy the laugh, but before it threatens to snowball into a series of gags you need to take command of the situation. You don't need to get nasty (don't punish the players for having a good time!) but you do need to be firm and to quickly place the focus back on the game. In your example above, it might take as little as you saying something like
''Alright, guys, Come on! - The bard emerges from the foliage, his faced flushed as he reaches the crescendo of his song. (turning to one of the players) Kevin, the orc lifts his club and swings for you head. What are you doing?''
If your players are at all reasonable, they should respond pretty well to this. In our games we enjoy a lots of jokes, but when the GM pulls the focus back on the game, we are all there.
Ah - I just realized you aren't the GM in this group. This makes it a little tricky, as it is the GM who sets the tone in a RPG. You could raise it with him (or just email him a link to this thread...)