If you mix at reasonable levels, nearfields shouldn't bother the neighbors at all. A lot of people listen to their stereos that loud.
I would recommend you also buy a Radio Shack Analog Sound Level Meter (about $40 these days) and check your playback levels often while mixing, both for your mix's sake AND your neighbors. There are much more sophisticated models, but they start at around $150 and climb quickly to well over $2000. The RS model is a great value for money.
Where most of the outward bound sound leakage occurs is trying to mic real drums, electric guitars with large amps or pushed sounds, and sometimes really enthusiastic singers with opera quality vocal chords. One of the blanket approaches mentioned should tame the vocal part, but the others (especially drums) would require very serious soundproofing. This is why a lot of HR guys use things like PODS, J-Stations, etc for guitars, and samples or drum machines for drums. They're not ideal by any stretch, but they keep us from going to jail or getting beat up, etc, by less appreciative types. The alternative is serious money and time thrown at the soundproofing, of which the real deal is never cheap. I'm not saying that the blanket tents and closets don't help, I'm only saying that they will only go so far. After that, it's time for serious construction either DIY or contracted; DIY requires more of your time but saves a lot of the cost, where hiring it done saves some of the time but opens up other cans of worms. (Who do you trust to know what they're doing, locally? What do you tell them you want? How much can you do, depending on whether you're in town or country, own or rent, have a wife that's demanding or understanding, yada yada yada - Now you see why so many PODS/Drum Machines and the like get sold...
In your case, the comment about the duplex suggests that you are renting. This can limit structural changes to zero depending on your landlord. Still, things like window inserts and door covers built from heavy materiels (dry-wall, MDF, particle board) and foam weatherstrip can help major leakage points such as doors and windows. Ingenuity can really help here.
Human ears cannot judge absolute sound level since we adapt to whatever the ambient noise level is. If you constantly listen to music at 95 dB, then that seems normal. But if you come from a quiet room into that sound field, it will seem loud (it is) until you get used to it. For this reason, as well as the uneven frequency response of the human ear depending on SPL, I strongly recommend using a SPL meter continuously when mixing. Among other things, it will help you develop consistent mixes that translate well to other systems.
I personally set up the RS meter for C weighting/fast, and always mix at an average of 85 dB with peaks never exceeding 90. This puts the SPL in a range that OSHA claims you can listen to all day without protection, places levels where the Fletcher Munson effects do the least damage to bass/treble perception, doesn't irritate neighbors with excessive bleed if your sound control isn't perfect, and maintains a constant reference volume.
When I mix, about every 2nd or 3rd tweak I do I re-check the playback level and correct it if necessary to maintain the 85 dB target. Doing this will save your ears, mix qualities, marriage, and public relations. Not bad for $40 or so... Steve
