Sometimes it is hard to use external usb card with amarok if it is not selected as default. In order to configure amarok to use usb sound card, you must learn the name of your usb sound card first with:
0 [Intel ]: HDA-Intel – HDA Intel
1 [U0x41e0x30d3 ]: USB-Audio – USB Device 0x41e:0x30d3
After that, just open the amarok settings window -> engine settings and checked output that if alsa is selected, if not select alsa first. And enter your soundcard’s name with a prefix of plughw: as below:
save the settings and play another file, it should work.