An update on this:

To begin, I installed alsa-plugins and alsa-plugins:pulseaudio and
configured the build of alsa-lib like this:

"--with-plugindir=/var/guix/profiles/per-user/leo/guix-profile/lib/alsa-lib"

Everything worked that way, but obviously it's not a solution.

Now, I am working on making alsa-lib respect ALSA_PLUGIN_DIRS, which
would be a search path specified by packages that provide ALSA plugins,
such as alsa-plugins. However, so far my attempt fails in another part
of alsa-lib, like this:

------
$ mpg123 -o alsa ~/file.mp3
ALSA lib dlmisc.c:204:(snd_dlsym_verify) unable to verify version for symbol conf_pulse_hook_load_if_running
ALSA lib conf.c:3686:(snd_config_hooks_call) symbol conf_pulse_hook_load_if_running is not defined inside libasound_module_conf_pulse.so
ALSA lib pcm.c:2685:(snd_pcm_open_noupdate) Unknown PCM default
[src/libout123/modules/alsa.c:181] error: cannot open device default
[src/libout123/libout123.c:455] error: Found no driver out of [alsa] working with device <default>.
main: [src/mpg123.c:314] error: out123 error 3: failure loading driver module
------

I don't know why snd_dlsym_verify fails with my patch but succeeds when
using '--with-plugindir'.

My current patch is attached...