Hi Bubinski,
You'll need to go into your SB512 directory and look for some kind of SB-properties icon. Click on it and you should find a tick box that enables simultaneous replay/record. Ensure that it's ticked.
Beware though! My AWE64 Gold was only half-duplex. I could only replay in 8-bit while recording 16-bit. It sounded dreadful so I never bothered with it. I bought myself a Terratec EWX 24/96 audiocard and it's way better than any SB card.
A lot of peeps use the SB Live! and have had good results. I'd still recommend an upgrade to a 24/96 audiocard though.
--
BluesMeister