Here's how to change the Raspberry Pi to use USB audio as the default.
This has proven quite hard to pin down online as things recently changed in the kernel, I think to use something called Device Tree Blocks.
You will find posts online suggesting you change:
But this method will work for all users, unlike changing ~/.asoundrc.
There is a setting which makes sure that any USB audio dongle you have plugged in is not the default. The device with the number of zero is the default.
And you will see a line which reads:
options snd-usb-audio index=-2
That is the line which keeps the USB audio device from being the zero default.
So, commenting it out like this:
#options snd-usb-audio index=-2
And then rebooting will make the USB device the new default.
Of course you will need to use sudo to do this.
Optionally you can also disable the Broadcom audio driver by commenting out a line in:
The line is:
Comment it out:
Now after rebooting you have USB audio and no Broadcom sound driver.
Note that assuming you don't disable the Broadcom sound driver and still want to hear some applications through either HDMI or the the analogue audio jack, you now need to force individual applications to use the on-board sound by providing whatever configs or switches each application needs.