Windows XP recognizes QEMU's emulated Intel 82801AA AC97 and Ensoniq AudioPCI ES1370 device automatically.
Intel 82801AA AC97 emulation works with Windows 7, but is detected as a "Multimedia Audio Controller" device. The correct driver can be installed through the Device Manager menu by selecting "Update Driver" in the device's Properties window. Windows will automatically obtain the correct driver from the internet.
ENSONIQ AudioPCI ES1370 emulation is also recognized by Windows 7 with the same process, but the driver is prone to crashing.