The default soundcard to be used with ALSA is that with the index 0 in
/proc/asound/cards. Usually this is the first
found sound device.
If you want to change this order, you have to find out the name of the kernel module used for that card and then add
index to it in a modprobe file. I appended these lines to my
options snd-hda-intel index=-2 options snd-usb-audio index=0
This way, the internal sound card (which was the primary one) now gets some other index and the USB mini speakers get the index 0 and therefore become the new default device for any sounds.