Turn off the mic input on your sound card

[boris81]

This is common sense but I think it merits a mention.

The microphone input is usually much noisier than the playback chain. I see about 20dB more noise compared to the line-in input and sometimes the signal can have additional boost. This is with no microphone actually plugged in, just noise in the bare circuit.

Some sound card drivers will mix the microphone input with the output. The easiest way to check this is to connect a microphone and see if your voice comes out of the speakers. Usually there will be a way to disable monitoring the microphone in the sound card's control panel. If there's no such convenient option I will advocate muting your recording inputs and disabling recording drivers through the OS.

Switching on the inputs while playing music produces an audible degradation in the sound quality for me.