I'm trying to install PulseAudio, but `apt-get` finds no installation candidate:

```
$ sudo apt-get install pulseaudio
Reading package lists... Done
Building dependency tree
Reading state information... Done
Package pulseaudio is not available, but is referred to by another package.
This may mean that the package is missing, has been obsoleted, or
is only available from another source

E: Package 'pulseaudio' has no installation candidate

```

Note that unlike [this question or its duplicates](https://askubuntu.com/questions/1035057/cannot-install-pulseaudio-after-upgrade-to-18-04), `apt-get` is *not* reporting any unmet dependencies.

`apt-cache` shows that versions are available, but there is no candidate:

```
$ apt-cache policy pulseaudio
pulseaudio:
  Installed: (none)
  Candidate: (none)
  Version table:
    1:11.1-1ubuntu7.5 -1
       500 http://au.archive.ubuntu.com/ubuntu bionic-updates/main amd64 Packages
    1:11.1-1ubuntu7 -1
       500 http://au.archive.ubuntu.com/ubuntu bionic/main amd64 Packages

```

So it looks like, unlike [this question](https://askubuntu.com/questions/1018602/e-package-apache2-has-no-installation-candidate-on-ubuntu-17-10), I can *see* the package I want, but I can't figure out why it can't be installed.

How can I resolve this?