The dilemma is that solving noise problems is an art within itself. Since it does not come up every day, we all have limited practical experience. This has spawned an industry for those who are now specialize in solving noise problems.
http://www.epanorama.net/documents/groundloop/index.html
http://www.musicfromouterspace.com/analogsynth/monomixerpage.html
http://www.musicfromouterspace.com/analogsynth/stereomixer.html
with Modulator
http://www.paia.com/9710scha.gif
It consists of 5 stereo inputs to plug in 5 stereo sound modules and a stereo out with volume controls.
http://europa.spaceports.com/~fishbake/mixer/linemix.htm
This simple line mixer allows mixing of two line level audio signals together to make on audio signal. This circuit can be used to mix the audo signal from for example two soundcards, CD player, casette deck or any combination of those.
http://www.epanorama.net/documents/audio/mixer.html
The mixer consists of two transistor-based preamplifiers. it works well with a regular dynamic microphone.
http://www.geocities.com/tomzi.geo/mixer/mixer.htm