Comment 5 for bug 46996

The only thing that can be done currently is to work around it by using /etc/modprobe.d/alsa-base. I'm not a fan of this approach, because it will silently break the semantics of current installs, but you can use:

options snd-usb-audio index=-1

Currently, the following is applicable:

Use case: I have an M-Audio Transit USB. I _want_ it to be the default sound device. It works that way on boot. I'm happy.

Use case: You have a USB Webcam. You _don't_ want it to be the default sound device. It doesn't work that way on boot. You're not happy.

Use case: Jane Doe has a USB MIDI device. She doesn't care whether it's default.

Now if we make the above change, presuming that the user doesn't already have an ~/.asoundrc* (or /etc/asound.conf), then my use case breaks (I'm not happy), your use case succeeds (you're happy), and Jane's use case still is moot.

In other words, it's a no-win situation particularly because forcing any USB sound device to a lower priority essentially necessitates the user having to use [something that invokes] asoundconf(1) [or writing out ~/.asoundrc* or /etc/asound.conf by hand], whereas formerly simply hotplugging the USB sound device after hardware drivers are loaded (fairly early in the boot sequence) "works".

Unfortunately, it looks like using asoundconf(1) is unavoidable for at least one of the use cases regardless which approach is used, so the /etc/modprobe.d/alsa-base approach will probably be made.