The Audio Selection Panel allows you to control 4 Transceivers and hear the audio through the built-in loudspeaker and on an external boomset (combination Microphone/ headset).
http://pe2er.nl/ASP/index.htm
This circuit is useful for removing ground loop hum on a remote line level audio signal line. It can be used to to connect a computer sound card to a stereo amplifier line input.
http://www.solorb.com/elect/musiccirc/stereoiso/index.html
http://www.musicfromouterspace.com/analogsynth/monomixerpage.html
http://www.musicfromouterspace.com/analogsynth/stereomixer.html
http://www.musicfromouterspace.com/analogsynth/stereoinputmixer.html
convert consumer equipment stereo signal to mono or combine audio signal from two different equipments to one input
http://www.tkk.fi/Misc/Electronics/circuits/linesum.html
http://www.epanorama.net/documents/audio/mixer.html
with Modulator
http://www.paia.com/9710scha.gif