Several key thoughts presented above, I'll paraphrase few of them:
The window simply needs to be dealt with. And with only one exterior wall, the specifications are a lot clearer. And if you have A/C, don't worry about heat.
For the window: It has to be "blocked" entirely. A "curtain" or window blind or anything, up against the glass. From the outside it should look like a normal interior window covering. However, it's up against the glass for a reason. That leaves you two or three inches of space to the edge of the inside wall. Here is where you need foam. The easiest thing to do is measure the inside on the window frame (height and width and also depth) and find a place that will cut a piece of foam to fit. I'm sure you can find an upholstery supply store.
So now you're left with one exterior wall (with a piece of foam the surface of which is level with the surrounding wall). The issue here is how much work is needed. As James mentions, test it. But how is the issue. One person making noise is a possibility. Or, if you happen to like horror movies, plug one in and turn it up while you're inside to a level that you've heard before somewhere. Then go outside and listen. Frankly, no one can let out a good scream that can match what the special effects guys can do. Perhaps the movie "Blow Out" with Nancy Allen's ending scream. Perhaps the gal in the original King Kong movie. The site "movieclips.com" has a half minute compilation of a bunch of the best ones, including guys, that could be set on replay (link below). I think Guinness has a loudest scream at maybe 120 dcb. But I digress. You'll need at least an acoustical decorative curtain across the entire wall. This in itself will allow guests comfort to let loose. Perhaps a layer of thin foam as well behind the curtain. It's the thickness of the foam, if any, that you need to test for. For the test, place a piece of curtain material, and a piece, or two, or three, of foam in front of the sound speaker, turn on the noise and go outside and listen. Consider mounting a 1x4 across the wall at both the floor and ceiling to fasten the top and bottom of the curtains to. Or perhaps 2x4s if you'll be attaching other things. If you do need to add a lot of foam, then perhaps some thin paneling over that, and then the curtain. Note that sound is an energy wave and it is actually directional. Thus, consider furniture positions in the room so that vocals are directed towards the interior of the house.
Back to horror movies, perhaps in a casual conversation with the neighbors you might accidently mention that you're a horror movie buff. Excellent cover in case some stray noise does get out. However, they might think it's ok to stop by before Halloween to ask your opinions on that topic
Couple other thoughts but this is to long already.
http://static.movieclips.com/embedplayer.swf?config=http://config.movieclips.com/player/config/embed/3xhs/%3Floc%3DUS&endpoint=http://movieclips.com/api/v1/player/test/action/&start=0&v=1.0.15