Comment 4 for bug 942780

The mono profile is usually thrown away as most hardware does not support opening it in one-channel mode only.

You can configure pulseaudio to do what you suggest by modifying
/usr/share/pulseaudio/alsa-mixer/profile-sets/default.conf
and
/usr/share/pulseaudio/alsa-mixer/profile-sets/extra-hdmi.conf

In both files, go to the [Mapping analog-mono] section and change
channel-map = mono
to
channel-map = mono,mono

This will take effect the next time pulseaudio is restarted (e g by rebooting).

The *major* question is however, whether we should do this by default, and if we risk regressing other hardware by doing so.