How to build your own                          
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This is a hypothetical design for a four channel random 'gate'
mixer. using a box with strips of conductive silverfoil -
4 for the inputs and 4 for the outputs, each next to eachter.

A bunch of metal balls can roll around freely in the box and
make random connections with the different audio signals when
the input strips connect with the output strips.

There is absolutely no use for this I guess, except it will be
fun and interesting for experimental music. You can also use
it for CV/gate signals in a modular set up or whatever as a
sort of random switcher.

You don't need any soldering or electronic engeneering skills to
make this, just some silverfoil, tape, wires, audio plugs and
a box...and ofcourse metal balls.

The result will be a strange conntraption you can shake around/
tilt to get random short audio connections.

You can use any number of channels we just chose 4 because
...why not. It is possible to make it with just 1 channel but
that might be a bit boring.

What you need:

- A box (wooden or plastic, cardboard is possible too but you
will be probably shaking it a bunch so you might want to have
something sturdy)

- Silverfoil - to make the strips.

- audio cable/wire

- Audio inputs for jack/minijack/RCA whatever you want to use
for this we need 4 inputs and one for the output, so 5 in total.

- Glue, tape  

- Metal balls - The balls have to be big enough so they can bridge
the gap between the input and output foils. You can have as many
balls as you want.  More balls will create a more chaotic and
intense sound. One ball will make a more sparse  minimal sound
because it makes only 1 connection at a time. 
You can make very good metal balls out of silverfoil, you might
know this as the Japanese silver foil ball challange were you just
roll a rough ball out of scraps of silverfoil and then keep adding
and rolling until you have a nice round ball.
Most probably this mixer will work better with a more 'sausage'
cilindrical shape, maybe try to experiment with different shapes and 
sizes of the metal balls to see what works best.
 
Now you can put everything together according to this SCHEMATIC:

                                            just knot or tape
           wires connected to               the wires together
            silverfoil strips ___                 |
                                 |                |     audio
             ____________________V________________V     output
            ||       _____________________________\     |O|
 metal balls||      ||       _____________________\     | |
roll around ||      ||      ||      ______________\     | |
   |  ______||______||______||_____||___          |      |
   | |      ||      ||      ||     ||   |         |______|
   | |  ||__||  ||  ||__||  ||  || ||   |
   |____\/  \|  ||  |/  \|  ||  || || <----- strips of silverfoil
     |  /    \  ||  /    \  ||  || ||   |
     |  \    /  ||  \    /  ||  || ||   |
     |  |\__/   ||  |\__/   ||  || ||   | <-- Box, probably handy
     |  ||  ||  ||  ||  ||  ||  || ||   |   if its enclosed so you can
     |  ||  ||  ||  ||  ||  ||  || ||   |   roll the metal balls around
     |  ||  ||  ||  ||  ||  ||  || ||   |   withouth having them fall
     |  ||      ||      ||      ||      |   out...or make a plexiglass
     |  ||      ||      ||      ||      |   lid so you can see them
     |_|O|     |O|     |O|     |O| _____|   roll around and have some
       input 1 input 2 input 3 input 4      visual feedback

       connect the individual audio inputs to the 'input'
       silverfoil strips with some wire and tape or you might
       want to solder it, as well as the 'output' silverfoil strips to
       the output wires.


If you are a littlebit more tech savy you can ofcourse make
volume pots/knobs on the channel so you can control the volume
of each channel individually.