Dell Precision 5750, on 20.04 the sound was working just fine.

But then, when I upgraded to 20.10, the system stopped recognizing my sound card, and a further upgrade to 21.04 didn't change anything. Force reloading/reinstalling alsa didn't help.

Sound settings say "Dummy output" as an output device.

Alsamixer only sees HDA Nvidia as a sound card for HDMI. So does "aplay -l":

**** List of PLAYBACK Hardware Devices ****

card 0: NVidia [HDA NVidia], device 3: HDMI 0 [HDMI 0]

  Subdevices: 1/1

  Subdevice #0: subdevice #0

card 0: NVidia [HDA NVidia], device 7: HDMI 1 [HDMI 1]

  Subdevices: 1/1

  Subdevice #0: subdevice #0

card 0: NVidia [HDA NVidia], device 8: HDMI 2 [HDMI 2]

  Subdevices: 1/1

  Subdevice #0: subdevice #0

card 0: NVidia [HDA NVidia], device 9: HDMI 3 [HDMI 3]

  Subdevices: 1/1

  Subdevice #0: subdevice #0

card 0: NVidia [HDA NVidia], device 10: HDMI 4 [HDMI 4]

  Subdevices: 1/1

  Subdevice #0: subdevice #0

card 0: NVidia [HDA NVidia], device 11: HDMI 5 [HDMI 5]

  Subdevices: 1/1

  Subdevice #0: subdevice #0

card 0: NVidia [HDA NVidia], device 12: HDMI 6 [HDMI 6]

  Subdevices: 1/1

  Subdevice #0: subdevice #0

"lspci -nnk | grep -A3 Audio":

01:00.1 Audio device [0403]: NVIDIA Corporation TU106 High Definition Audio Controller [10de:10f9] (rev a1)

    Subsystem: Dell TU106 High Definition Audio Controller [1028:0990]

    Kernel driver in use: snd_hda_intel

    Kernel modules: snd_hda_intel

01:00.2 USB controller [0c03]: NVIDIA Corporation TU106 USB 3.1 Host Controller [10de:1ada] (rev a1)

"dmesg | grep -i audio":

    [    0.171470] ACPI: Added _OSI(Linux-Lenovo-NV-HDMI-Audio)

    [    4.994502] snd_hda_intel 0000:01:00.1: Handle vga_switcheroo audio client

    [    5.018582] sof-audio-pci 0000:00:1f.3: DSP detected with PCI class/subclass/prog-if info 0x040100

    [    5.018614] sof-audio-pci 0000:00:1f.3: enabling device (0000 -> 0002)

    [    5.018803] sof-audio-pci 0000:00:1f.3: DSP detected with PCI class/subclass/prog-if 0x040100

    [    5.018860] sof-audio-pci 0000:00:1f.3: bound 0000:00:02.0 (ops i915_audio_component_bind_ops [i915])

    [    5.023774] sof-audio-pci 0000:00:1f.3: use msi interrupt mode

    [    5.038285] sof-audio-pci 0000:00:1f.3: hda codecs found, mask 4

    [    5.157409] sof-audio-pci 0000:00:1f.3: Firmware info: version 1:5:1-88707

    [    5.157413] sof-audio-pci 0000:00:1f.3: Firmware: ABI 3:16:0 Kernel ABI 3:18:0

    [   64.498859] sof-audio-pci 0000:00:1f.3: DSP detected with PCI class/subclass/prog-if info 0x040100

    [   64.499051] sof-audio-pci 0000:00:1f.3: DSP detected with PCI class/subclass/prog-if 0x040100

    [   64.499130] sof-audio-pci 0000:00:1f.3: bound 0000:00:02.0 (ops i915_audio_component_bind_ops [i915])

    [   64.505167] sof-audio-pci 0000:00:1f.3: use msi interrupt mode

    [   64.516834] sof-audio-pci 0000:00:1f.3: hda codecs found, mask 4

    [   64.660436] sof-audio-pci 0000:00:1f.3: Firmware info: version 1:5:1-88707

    [   64.660440] sof-audio-pci 0000:00:1f.3: Firmware: ABI 3:16:0 Kernel ABI 3:18:0

"lsmod | grep snd_hda_intel":

snd_hda_intel          53248  1

snd_intel_dspcfg       28672  3 snd_hda_intel,snd_sof_pci,snd_sof_intel_hda_common

snd_hda_codec         147456  3 snd_hda_codec_hdmi,snd_hda_intel,snd_soc_hdac_hda

snd_hda_core           94208  7 snd_hda_codec_hdmi,snd_hda_intel,snd_hda_ext_core,snd_hda_codec,snd_sof_intel_hda_common,snd_soc_hdac_hda,snd_sof_intel_hda

snd_pcm               118784  14 snd_hda_codec_hdmi,snd_hda_intel,snd_hda_codec,soundwire_intel,snd_sof,snd_sof_intel_hda_common,snd_sof_intel_ipc,snd_soc_rt715,snd_compress,snd_soc_core,snd_soc_rt711,snd_soc_rt1308_sdw,snd_hda_core,snd_pcm_dmaengine

snd                    94208  13 snd_seq,snd_seq_device,snd_hda_codec_hdmi,snd_hwdep,snd_hda_intel,snd_hda_codec,snd_timer,snd_compress,snd_soc_core,snd_pcm,snd_rawmidi

So I surmise that my card is an onboard Intel Realtek something, and that it should use the snd_hda_intel driver. I've also double-checked that it was switched on in BIOS.

Any ideas where to look next?