One of the nuances of running multiple sound cards on Linux is that, at least in Archlinux, the order in which kernel sound modules are loaded determines which of the system’s multiple sound cards will be assigned to /dev/mixer, /dev/mixer1, etc.
In Archlinux the MODULES array in /etc/rc.conf is a global array that can be used to specify which and in what order to load kernel modules.
For example, this
MODULES=(snd_usb_audio snd_intel8x0)
would load my Extigy USB sound card before my onboard intel card. Similarly, this
MODULES=(snd_usb_audio !snd_intel8x0)
disables my Intel card. In both of the above examples the USB card would be assigned to /dev/mixer.
On the other hand, reversing the order assigns the Intel card /dev/mixer
MODULES=(snd_intel8x0 snd_usb_audio)