Hi nurit and welcome to TSF
Apologies for not replying earlier - The 'Stero Mix' or 'What I hear' feature is controlled by the driver-software, rather than the card itself. Unfortunately, it's also a feature that's now become rather rare since Win-7.
It was common in Win-XP, but for some reason, the Win-7 updated drivers haven't included it. (The same card in my PC (Dual-boot (XP~Win-7) suffers the same problem)
Oddly, some of the 'on-board' sound-chips (usually 'Realtech') have the 'What-you-hear' feature in their software, I think it's just in the 'High Definition' chips though, I don't have a HD-chip on my mobo so I can't say for sure.
'Creative' have an excellent range of USB sound-cards to fit most budgets, it seems to be 'pot-luck' whether you get the 'Stereo-Mix features though