1. Open the sound recorder
2. Check the list "Record from input". In my case I see:
   Line-in
   CD
   Microphone
   Video
   Phone
   Aux
   Capture
3. Start to record.  I leave the default (CD)
4. Stop it after a seconds.
5. Start to record again.
6. Stop it after a seconds.
7. Check the list "Record from input"

Now the list its double long, with the same list in 2. but twice.

You can repeat 6 and 7 any time you want, and the list will increase its size in each step.