On Ubuntu (and possibly other linux distributions) Miro refuses to work with a secondary sound card, it will only work with the primary one despite what the ALSA default is set to, unlike most programs which offer some way to override the default.

Potentially, the second sound card in question could be a PCI card or something else, but based on other people’s experience (like my own) it is usually a USB sound card that is causing trouble. See here (note, the suggested fix didn’t work for me, just like it didn’t the original poster there) and here (they mention fixing it in the trunk, but that doesn’t help me until a new release comes out).

Some people actively want both the onboard sound and the USB or PCI device working, but if you are willing to sacrifice on-board sound, I found a work around. In my case, the on-board sound is worthless. It has some terrible humming/buzzing in the background so I never ever want to use it again.

The solution is to find what the module is that supplies your on-board sound. In my case, the on-board sound is a VT8233, so when I looked at the output from lsmod, it was obvious that the module for this sound device was the snd_via82xx module.

Then, open the /etc/modprobe.d/blacklist file to edit it:
sudo pico /etc/modprobe.d/blacklist
and add the line:
blacklist snd_via82xx
Then reboot.

Now, the USB audio device will be the first audio device.