I have the same issue. Sound works in games etc. because I have configured the defaults to point to my USB hardware, but volume level changes fail and related errors persist. I think it’s because of calls to the simple channel element “PCM” which is no longer available when I enable my USB audio as the default. In my case, it’s a C-Media adapter that responds to the channel element named “Headphone”.
One promising I idea I was trying was to setup an ALSA plugin that makes my USB hardware mirror everything sent to the built-in audio. This requires undoing the USB default settings so the built-in audio becomes the default again and the “PCM” named element is available. That eliminated the errors, but not every emulator had audio. If we could make this audio syncing work with some ALSA expertise, that would solve the issue for everyone, regardless of their USB card’s channel element names. It’s beyond my knowledge of ALSA setup–the trial and error takes so long because I don’t know what exactly I am doing!