For HDMI video to work with our drivers likely depends on the hardware/firmware and seems to usually work though Paul reported only one of his 2 HDMI ports worked for him.
Display Port is similar to HDMI and only needs a passive connector to convert.
As the audio is carried on the same wires as the video. On Display Port it is inserted into the video blanking part of the signal. Video blanking on a CRT was when the electron beam shut off and went from the bottom of the screen to the top to draw the next frame, so a brief pause in video every frame. Games and such (our Firefox is supposed to but doesn't) also use that period for doing calculations etc.
USB C has an alternative mode which amongst other things, can act like a Display Port cable, this is how monitors are hooked up via USB 3+, so not really USB, but rather Display Port (or HDMI) transmitted over the USB C cable. Sound would work the same as normal Display Port and likely HDMI, multiplexed into the video signal, so the video hardware/sofware needs to be involved. This is different from USB audio that Lars works on by my understanding.
HDMI audio seems quite diverse according to wiki, it is all optional but to quote wiki,
https://en.wikipedia.org/wiki/HDMIFor digital audio, if an HDMI device has audio, it is required to implement the baseline format: stereo (uncompressed) PCM. Other formats are optional, with HDMI allowing up to 8 channels of uncompressed audio at sample sizes of 16 bits, 20 bits, or 24 bits, with sample rates of 32 kHz, 44.1 kHz, 48 kHz, 88.2 kHz, 96 kHz, 176.4 kHz, or 192 kHz.[5]: §7 HDMI also carries any IEC 61937-compliant compressed audio stream, such as Dolby Digital and DTS, and up to 8 channels of one-bit DSD audio (used on Super Audio CDs) at rates up to four times that of Super Audio CD.[5]: §7 With version 1.3, HDMI allows lossless compressed audio streams Dolby TrueHD and DTS-HD Master Audio.[5]: §7 As with the Y′CBCR video, audio capability is optional. Audio return channel (ARC) is a feature introduced in the HDMI 1.4 standard.[41] "Return" refers to the case where the audio comes from the TV and can be sent "upstream" to the AV receiver using the HDMI cable connected to the AV receiver.[41] An example given on the HDMI website is that a TV that directly receives a terrestrial/satellite broadcast, or has a video source built in, sends the audio "upstream" to the AV receiver.[41]
Arca Noae does have updated video drivers (Intel at first) on their roadmap, when they find time I guess. At that point they may well add audio support to HDMI and Display Port.
BTW, my 1900x1200 monitor looks much better then VGA (mostly sharper, especially full screen text like during boot) using the Display Port and a DVI converter to hook into the DVI monitor plug. It is slightly broken compared to VGA, X doesn't see the VGA bios and while the screensaver can turn off my monitor, it won't turn back on except by rebooting.