Macintosh OS X

1. Ope System Preferences
2. Click Sound
3. click Output
4. Click all of them and Mute the, but the headphones. Make sure they are not muted.
5. Verify that if you change the volume, you hear a plip sound through the headphones.
6. Then click Input.
7. Select the headset. Speak ... verify that the lighty-uppy things light up.
8. Adjust the volume so that when you speak, it's about 2/3 to max.
9. Now in SL.
10. splat-P for preferences, Voice chat tab
11. click device settings
12. Form Input Device list, choose the right thing
13. From Output Device settings choose the right thing
14. Speak ... you should see the green lights light up
15. Click the X.
16. You should see "Connecting to in-world Voice Chat... Connected"

Windows

If you have a similar procedure for Windows, please write it up.