If you need this feature, you may try OSS4
With OSS4, you can play all your audio apps simultaneously
and mix sound in virtual mixer (vmix).
If your soundcard is not supported by OSS4, you are out of luck.
You may also try an advanced solution:viewtopic.php?f=48&t=32725
Notice that the advanced solution was designed for 32 bit, fresh clean install.
The point is to ensure that all the right things are on the place.