I am trying to use Ardour with an external USB mixer (Alesis Multimix4) in Xubuntu.
In the xfce sound settings, I can turn off the internal sound card and use the USB mixer instead as an I/O. This works for all desktop applications. However, Ardour ignores this and continues to use the laptop’s built-in sound card.
As said before, you will have better luck to start Ardour with Jack. Actually, you have to ! So, erase completely Pulseaudio before install Jackd (there’s howto in Ubuntu forums, and in Linux musicians site)