If you're choosing "Soundcard Input" and people can no longer hear the music, it means your soundcard's volumes have not been set correctly.
You will need to find someone who can listen while you test your levels and remember,
there will be a delay between what you try on your computer and what the listener hears so be patient.
I can give you some starting points, but you'll have to tweak your own settings.
1) Open your Winamp and click on the "Input" tab.
2) Choose Soundcard Input under "Input Device".
3) Choose 44,100Hz Mono, or Stereo from the "Input Settings"...I do not recommend 22,050.
4) Make sure Microphone is checked under the "Soundcard Mixer - Mic Input".
5) Set the level sliders as follows:
Music Level = -2dB
BG Music Level = -11dB
Mic Level = -3dB
Fade Time = 500ms
6) Click on the "Open Mixer" button...This will open up two windows of volume sliders.
7) On the "Play Control" window, set your "Play Control" volume between the second and third hash mark from the top. Also set your "Wave" and "CD Player" volumes to the same.

Still on the "Play Control" window, click the "Advanced" tab underneath the "Microphone" volume and put a checkmark in the box that says "Microphone +20dB boost".
9) On the "Recording Control" window, set the "What U Hear" (Stereo Control on some soundcards) between the third and fourth hash marks...Make sure that
only the "What U Hear" or "Stereo Mix" box is checked...
Do not check any other boxes.
10) Still on the "Recording Control" window, set your Microphone volume between the second and third hash marks. Click on the "Advanced" tab and make sure the "Microphone +20dB boost" is checked hear as well.
11) Load up some music and watch the "Input Levels" of your SHOUTcast Source. They should max out between
-3dB and -2 dB. Never let these values be higher since it can cause major clipping and distortion noise on the listener's end.
You will, most likely, have to tweak the above settings to suit your own system since each computer/soundcard set up is different. Just remember to be patient and try not to get frustrated. This is a fairly time consuming process in most cases.
On a side note,
never....I repeat....
never fiddle with graphic equalizer settings on the computer that's broadcasting. Always leave these values at 0 (zero). What may sound fantastic on your own system may overload a listener's sound set up and sound like complete garbage.