The circuit was designed to provide an overall gain of one between the output and each input channel on an audio mixer with 3-channel input. 

A mixer is a device that can be used to merge two or more electronic signals into one complex output. It can have a classification as frequency mixer, electronic mixer, audio mixing console, mode mixer, or vision mixer. 

A single capacitor with 0.1uF value is attached in every input channel along with a 100K Ohms resistor in order to provide 100K Ohms output impedance. Adding more capacitors and resistors with the same values of 0.1uF and 100K Ohms respectively, will increase the number of input channels to this audio mixer. The proper location of the mixer os situated between the input to the power amplifier and the output of the tone control circuit. The other inputs should either be grounded or taken from the desired source while one input should be taken from the output of the tone control circuit. 

The transistor 2N3904 was designed for general purpose low power amplifier and switch applications which can extend to 100mA as a switch and to 100MHz as an amplifier.