How can I record stereo on my laptop (see details) ?

check whether this helps.

http://audacity.sourceforge.net/help/faq?s=recording&i=stereo

An exeternal USB soundcard would probably be the best solution. Creative have a range that starts from £30.

re: External USB sound card

I use a Griffin iMic, about $24 US, it works great.
http://www.griffintechnology.com/products/imic/

Other USB adapters start below $10 US. I don’t know if the Griffin piece has any advantage besides the ability to take mic level input as well as line input.

They both do output, too, and I have found the iMic sounds better than built-in internal sound cards.

Either way, no drivers are needed. Windows XP and above, OS X and Linux all recognize the hardware.

Thanks to all who answered!
@tuxuday In this case the problem is a deficiency of hardware, not just a software setting. Interestingly the mic input is routed to the left channel—usually the right channel is the default for mono.
@OreetCocker That’s apparently the kind of product I need. The Creative’s device is well-reviewed but I decided to go with the Griffin.
@jaytkay Yes, thanks for the recommendation. I heard good things about it & ordered it from Amazon for $24.20.