I works differently (from my experience from the past).
MCI_SYSINFO_QUANTITY, asking for MCI_DEVTYPE_AUDIO_AMPMIX will return to you as many devices as you have sound drivers installed (simplified way of saying). That's because each audio driver installation will install one MCI_DEVTYPE_AUDIO_WAVEFORM_AUDIO device (and/or potentially a MCI_DEVTYPE_SEQUENCER for MIDI) and also one MCI_DEVTYPE_AUDIO_AMPMIX device (the latter was added late in Warp 3 in order to support DART and new mixing functionality and you have to have such a device in order to play sound at all).
Therefore if you install both USB audio drivers (USBAUDIO.SYS and USBAUD2.SYS) you will be reported back two MCI_DEVTYPE_AUDIO_AMPMIX devices. If you additionally installed UNIAUD, you'd get reported three MCI_DEVTYPE_AUDIO_AMPMIX devices. You cannot tell from that response what is the default ampmix (waveform audio) device.
If you want to identify the default audio device (which is equivalent to identifiying the default ampmix device), then you will need to use MCI_SYSINFO_ITEM / MCI_SYSINFO_QUERY_DEFAULT and specify in the MCI_SYSINFO_DEFAULTDEVICE usDeviceType = MCI_DEVTYPE_AUDIO_AMPMIX.
Please note that the default ampmix (and wave audio) device would need to be set in MMPM2.INI which either happens "by hand" or better by either the selection utility that comes with the USBAUDIO installation (integrated into the Multimedia Object, look there) or by the automatic default audio device selection tool that comes with ArcaOS.
SDL2 correctly wants to enumerate all known devices. The weird thing is, that it always shown two devices with the same name, even when UNIAUD is installed and the USB headset is plugged in. I switched the default setting on the Multimedia Object, but I don't seem to recall the automatic selection tool on ArcaOS, you mentioned. Where is that located?
I'm pretty positive, that I only have usbaudio.sys in config.sys, but I would have verify it, if I have access to my machine again.