Updating the BIOS seemed to trigger at least partial recognition - the 'no codecs' message went away but ALSA didn't seem happy and no soundcards shown through 'aplay -l'. (I hadn't considered this previously as it had worked under Debian and none of the kernel release notes seem to suggest an abandon of older BIOSes.)
I also decided to switch from ASCII to Beowulf sources and do a full dist-upgrade. Although I had previously tried the Beowulf kernel (4.16) over ASCII (4.9) I thought best to get the latest versions of ALSA et al and a consistent level so that if there were any bugs or patches then I'd be in a better place to receive them.
(not needed in the end but https://ubuntuforums.org/showthread.php?t=2232882 looked useful for fault-finding similar)

I'm not sure which or all of the above was the magic, but that got me to a place where 'aplay -l' as root would detect the soundcard.
A bit more searching https://unix.stackexchange.com/question … -by-kernel revealed that my user had somehow got removed from the 'audio' group during all the fun and upgrades so 'aplay -l' as user had no soundcards. A bit of frustration that adding the user to audio didn't seem to persist for the first couple of logouts/logins but some carefully directed cursing at the computer eventually resolved that.

I'm going to give it 24 hours before marking as solved but I am happily dancing to music as I type!

I've probably installed some unnecessary firmware-* packages in the process and I'll look to carefully remove those once everything else is stable as it would be nice to have minimal 'non-free' but I'm not going to tamper with that today.

grep dmesg with either 'hda' or 'codec' only shows a single entry

[    9.239822] snd_hda_intel 0000:00:1b.0: no codecs initialized

I see. I am out of ideas then i guess. Hopefully someone else has a better grip on the topic. Model option might be still worth a try even if just to specify a generec Xjack model. Also for reference this was what i was looking at concerning the codec detecting problem: https://01.org/linuxgraphics/gfx-docs/d … ng-problem.

snd_hda_intel          36864  0
snd_hda_codec         135168  1 snd_hda_intel
snd_hda_core           86016  2 snd_hda_intel,snd_hda_codec
snd_hwdep              16384  1 snd_hda_codec
snd_pcm               110592  3 snd_hda_intel,snd_hda_codec,snd_hda_core
snd                    86016  5 snd_hda_intel,snd_hwdep,snd_hda_codec,snd_timer,snd_pcm

grep dmesg with either 'hda' or 'codec' only shows a single entry

[    9.239822] snd_hda_intel 0000:00:1b.0: no codecs initialized

Thanks for the alsa link; I couldn't instantly see anything that shouts of mine but will have a proper look later.

but lsinitramfs only shows

lib/modules/4.9.0-6-amd64/kernel/sound
lib/modules/4.9.0-6-amd64/kernel/sound/soundcore.ko
lib/modules/4.9.0-6-amd64/kernel/sound/core
lib/modules/4.9.0-6-amd64/kernel/sound/core/seq
lib/modules/4.9.0-6-amd64/kernel/sound/core/seq/snd-seq-device.ko
lib/modules/4.9.0-6-amd64/kernel/sound/core/snd-rawmidi.ko
lib/modules/4.9.0-6-amd64/kernel/sound/core/snd.ko

when I think I should be seeing some 'snd-hda-intel' entries in there?

Don't think thats the problem. I've just checked my initrd and it also just contains those files while my intel hda card is working fine.

lsmod | grep hda shows the following modules as loaded:

snd_hda_codec_hdmi     49152  1
snd_hda_codec_conexant    24576  1
snd_hda_codec_generic    69632  1 snd_hda_codec_conexant
snd_hda_intel          36864  0
snd_hda_codec         135168  4 snd_hda_intel,snd_hda_codec_conexant,snd_hda_codec_hdmi,snd_hda_codec_generic
snd_hda_core           86016  5 snd_hda_intel,snd_hda_codec_conexant,snd_hda_codec,snd_hda_codec_hdmi,snd_hda_codec_generic
snd_hwdep              16384  2 snd_hda_codec,snd_usb_audio
snd_pcm               110592  5 snd_hda_intel,snd_hda_codec,snd_usb_audio,snd_hda_core,snd_hda_codec_hdmi
snd                    86016  15 snd_hda_intel,snd_hwdep,snd_hda_codec_conexant,snd_hda_codec,snd_usb_audio,snd_timer,thinkpad_acpi,snd_rawmidi,snd_hda_codec_hdmi,snd_hda_codec_generic,snd_usbmidi_lib,snd_seq_device,snd_pcm

Looking around a bit problems to get the available codecs seems to be a known. Maybe grep dmesg for hda_codec if there is anything suspicious. Seems that's also related to getting the right card model which is what i was referring to by intel hda module options (see: http://git.alsa-project.org/?p=alsa-ker … xt;hb=HEAD).

I've got
   firmware-linux
   firmware-linux-free
   firmware-linux-nonfree
   firmware-misc-nonfree
   firmware-realtek   [network adaptor]
   firmware-intel-sound
all installed.

modprobe snd-hda-intel

gives nothing which I think means okay(?)

but lsinitramfs only shows

lib/modules/4.9.0-6-amd64/kernel/sound
lib/modules/4.9.0-6-amd64/kernel/sound/soundcore.ko
lib/modules/4.9.0-6-amd64/kernel/sound/core
lib/modules/4.9.0-6-amd64/kernel/sound/core/seq
lib/modules/4.9.0-6-amd64/kernel/sound/core/seq/snd-seq-device.ko
lib/modules/4.9.0-6-amd64/kernel/sound/core/snd-rawmidi.ko
lib/modules/4.9.0-6-amd64/kernel/sound/core/snd.ko

when I think I should be seeing some 'snd-hda-intel' entries in there?

I guess next step is experimenting with grub boot settings (which I'm not looking forward to). Just seems odd the sound loss and codec error appeared in the Debian -> Devuan transition so I was hoping it was more a package that had got missed / incorrectly removed in that change and therefore a quick fix.

On bootup / dmesg I get:

snd_hda_intel 0000:00:1b.0: no codecs initialized

Motherboard is:
    Manufacturer: Gigabyte Technology Co., Ltd.
    Product Name: H77N-WIFI

aplay -l
aplay: device_list:270: no soundcards found...

cat /proc/asound/cards
--- no soundcards ---

From searching on the "no codecs initialized" the general consensus seems to be to update the kernel so I've tried latest 4.16 (Beowulf / Ceres) and that made no difference so have reverted to 4.9 (Ascii). Grateful for any pointers as to what to try next, please.