2 Input Mixer:

2 Line Output Combiner: Convert consumer equipment stereo signal to mono or combine audio signal from two different equipments to1-input  

4 INPUT MIXER: This is a very simple 4 input audio mixer using op amps, potentiometers resistors, decoupling caps on the inputs and the whole unit has a balanced output good little circuit. 

6 Input Mixer: 3 line inputs and3 mic inputs  

Advanced Mixer Design: good article on large audio mixer design, read also Noise & Headroom in Mixers  

Audio Mixer:

Ever Amplifier Bridging: a design for a simple add-on bridging adapter for stereo power amplifiers  

FET Audio Mixer:

High Quality Sound Mixer: very ambitious and expected to be very expensive mixer project, modular design  

line Mixer: mix two line level sources to one-signal, includes separate volume controls for both signals, passive circuit  

Microphone Mixer: Designed for3-dynamic microphones  

Six Channel Mixer and Amplifier:

Sound Mixer/Amplifier:

Stereo Mixer : Can mix four separate audio inputs to mono or stereo output signal 

Two Input Mixer:

Two Line Output Combiner: Convert consumer equipment stereo signal to mono or combine audio signal from two different equipments to1-input  

Unbalanced Mute Circuit for Audio Mixing Channels: application note in PDF format  

Voice Over Circuit: Switches automatically from sound source to microph1-when enough sound comes in from microphone. 

Voltage Controlled Mixer with Modulator: