"Dummy Output" instead of audio device on Debian 9

In my case the problem was caused by the fact that another service (namely, timidity, started at the system level) was using the sound card, so pulseaudio (started at the user level) was not using it.

To check this, you can do sudo fuser /dev/snd/* and see if another process is using the sound card. If yes, you should arrange for the process not to start, and this will fix the problem.

To give more details, I was able to isolate the problem by editing /etc/pulse/daemon.conf to set log-level = debug, restarting pulseaudio with pulseaudio -k (then pulseaudio is automatically respawned by systemd), and this is how I noticed the problem:

Aug  7 11:37:34 zeta pulseaudio[22178]: D: [pulseaudio] module-udev-detect.c: /devices/pci0000:00/0000:00:1f.3/sound/card0 is busy: yes