Re: mixing audio channels & seperating them at the targe
There are several options.
The easiest is usually to not mix the audio and
instead install a multipair cable.
If you really want to combine six or twelve audio channels then there are a few options.
There is various equipment you can buy off the shelf to do this but it's expensive. Google for "digital snake" to find studio/theatre/band/concert equipment.
The main choice here is analog or digital.
The analog way is to use frequency modulated carriers with a different carrier frequency for each channel. If you want high quality and or to support a wide range of volume then it is is hard to get good linearity and low noise.
You probably need to use non harmonically related carrier frequencys to minimise the requirement for spectrally pure carriers and receiver performance.
The digital way to digitise the audio channels and combine that data into one stream. Sending high speed data has it's own problems of needing cable that can carry high frequencys, low crosstalk and interference, impedance matching etc.