System Administration Guide: Devices and File Systems

Hot-Plugging Multiple USB Audio Devices

If a USB audio device is plugged into a system, it becomes the primary audio device, /dev/audio. It remains the primary audio device even after the system is rebooted. If additional USB audio devices are plugged in, the last one becomes the primary audio device.

For additional information on troubleshooting USB audio device problems, see usb_ac(7D).