This popped up after fixing  FS#69992  but might manifest on other setups as well.

The kernel defines max_alloc_per_card = 32MB (pcm_memory.c:30);
now, take into account SND_HDA_PREALLOC_SIZE=4096 as per Arch Linux default and a typical Kabylake laptop with, for the sake of argument, 10 audio frontends (DMIC, HDMI, Analog out, Analog in):
FEs * SND_HDA_PREALLOC_SIZE * 1024 = 10 * 4096 * 1024 = 42MB > max_alloc_per_card
which results in the system exhausting the preallocated memory and (silently!) failing to allocate frontends (in my case, HDMI3 and DMIC).

Now, this can be worked around by skipping preallocation and setting SND_HDA_PREALLOC_SIZE = 0.
However, this causes some applications to allocate all memory available and end up with -ENOMEM.
Users can fix this by overriding max_alloc_per_card via modprobe, setting it to an appropriate value (in my case, 41943040). But this assumes SND_HDA_PREALLOC_SIZE = 4096.

Discussion is ongoing[1], the ALSA mantainer proposed the following temporary fix:
- set SND_HDA_PREALLOC_SIZE to 0
- set AZX_MAX_BUF_SIZE to something sensible, like 4MB (right now it's ~1GB)

1: https://bugzilla.kernel.org/show_bug.cgi?id=201251