I think it's a limitation of the emulation. The AC97 device is usually configured to behave like a SoundBlaster PCI 128, by the driver supplied with the mobo. It probably answers to the same addresses, too. If you want to plug in a real card, you have to disable it in hardware (it makes it easier for the driver to ignore it).
In your case you are using an SB Live!, which won't be confused with a PCI 128, so there is no conflict.
I don't know how it's done on an A1 (yet).
tony