Multiple audio device setup on OpenBSD 7.9 with Firefox

I am trying to make Firefox use a USB microphone (Razer) for Discord
voice chat, while also outputting all sound to a second USB sound device
(SMSL).

The Razer can also function as a playback device, but I am not
connecting any speakers to it. I am only using the SMSL to hear all
playback.

With both devices connected, here is the output of dmesg | grep 'audio':

uaudio0 at uhub4 port 1 configuration 1 interface 1 "Razer Inc Razer Seiren X" rev 1.10/1.00 addr 3
uaudio0: class v1, full-speed, sync, channels: 2 play, 2 rec, 10 ctls
audio0 at uaudio0
uaudio1 at uhub4 port 3 configuration 1 interface 1 "SMSL SMSL USB AUDIO" rev 2.00/3.53 addr 7
uaudio1: class v2, high-speed, async, channels: 2 play, 0 rec, 0 ctls
audio1 at uaudio1
uaudio0: rec xfer, err = 6
uaudio1: sync play xfer, err = 6
uaudio1: sync play xfer, err = 6
uaudio1: sync play xfer, err = 6
uaudio1: sync play xfer, err = 6

I'm assuming that the errors at the bottom aren't going to get in my
way, but I could be wrong I suppose.

What's the optimal way to configure sndio/mixerctl/etc. to do this?

I'm running the 7.9 release.