As a long time sporadic guitarist in various bands, I've faced the recurring issue of rehearsals being impaired by noise complaints. Particular post kids, the only time free to rehearse was often after 9pm and in my living room, not very band friendly!
I wanted a way of practicing with minimal noise, but without sacrificing clarity. I looked at headphones split via a headphone amp/mixer from a mixer desk but while the overall volume could be controlled per individual, we all had to work with the same mix. Then I came across matrix mixers and in particular, plans for a passive matrix mixer (no power req! and infinitely safer for a novice solder monkey). The matrix mixer allows each member to input and control their own mix of each of the members while listening to the output through headphones. Silent rehearsals (or at least as quiet as the sound of an unplugged electric guitar, the faint tap of an electronic kit or unamplified vocals).
Step 1: The Passive Matrix - 4x4
First the disclaimer - I am not an electronics expert (in fact very limited knowledge or skills) and recommend you seek good advice before playing with electronics.
The passive matrix mixer consists of 100K volume pots, female 3.5mm jacks, 15K resistors, a case, wire, and a fair bit of repetitive soldering. For a schematic, have a look at http://ilovefuzz.com/viewtopic.php?f=151&t=49681
for an excellent discussion and schematic options.
Basically the make up of the unit is that each input is split to each of the mixing sets (e.g. 4 inputs x 4 output channels) that are then mixed down to an output. As this is a passive (not amplified) unit, the resulting output can be low, this can be resolved with an inexpensive headphone amp (battery operated) for each output.
You can even create a "stereo" input and output by using double ganged (stereo) pots although beware, it involved double the soldering and is a bit more fiddly.
Overall, it is a fairly simple and relatively inexpensive (under $80AU) project to put together that we have used for rehearsals anywhere and anytime, even sitting unpowered out in the bush using iPhones and iPads as our amp models/synths. I hope it is helpful!