First make sure you can see the alsa card for the USB audio device with
And look for something that says "USB Audio" or similar
Depending on whether you are running a Pi 4 or other, it may be device 1, 2 or 3.
we used to recommend manually setting the device number to be your USB audio card but that turned out to be fragile because the HDMI audio device numbers would change.
So now we suggest just disabling the built in headphone jack audio, and letting the USB audio card be 'first'
Start by disabling the built-in headphone jack with
sudo nano /etc/modprobe.d/raspi-blacklist.conf
and typing in
hit return, then save with Control-X then Y then Return (to save)
Then enable the USB audio card with
sudo nano /lib/modprobe.d/aliases.conf
find the line with
options snd-usb-audio index=-2
and put a # in the beginning of that line. Then exit and save file.
sudo reboot to 'set' the changes
Upon reboot, if you run sudo aplay -l to list the interface, the USB audio card will be first and the default!