I've just installed Devuan Beowulf 3.1 with the Mate desktop and experienced the same issue – the settings are not being saved.  Here's my workaround.

Like me, you might have more than one sink.  Usually, there is only one.  To check, run:

pacmd list-sinks | less

Note the index of the sink that you want to be the default.  Do the same for sources:

pacmd list-sources | less

Make a backup of /etc/pulse/default.pa.  Open the file as root with a text editor, for example:

sudo vi /etc/pulse/default.pa

At the end of the file, there is a two-line option to make some devices default:

### Make some devices default
#set-default-sink output
#set-default-source input

Uncomment the two lines (by deleting the hashes) and set the output to the correct index for the sink and the source.  In my case, it was:

### Make some devices default
set-default-sink 0
set-default-source 0

If you found the output of pacmd hard to interpret, note that there are only two possibilities if you have two sinks (0 and 1), so you can experiment.  I had three sources, so three possibilites: 0, 1, 2.

Devuan's /etc/pulse/default.pa has an option that restores the default sink/source if changed by the user, which might be part of the problem.  I commented it out:

### Automatically restore the default sink/source when changed by the user
### during runtime
### NOTE: This should be loaded as early as possible so that subsequent modules
### that look up the default sink/source get the right value
#load-module module-default-device-restore

There's a similar option that restores the volume, which you might want to look at.

### Automatically restore the volume of streams and devices
load-module module-device-restore
load-module module-stream-restore
load-module module-card-restore

Hope this helps someone else!