Preferentially using an external USB Audio Interface as a primary/default pulseaudio sink when available.

On my Lenovo laptop, there is an internal Intel HDA sound card, but a far superior TASCAM audio interface is plugged into my dock, which is in turn wired up to my speakers. When docked, I want the laptop to use the TASCAM sound card, but when roaming, I want it to switch to the internal.

entropy@symplex:~$ pactl list short cards
0 alsa_card.pci-0000_00_03.0 module-alsa-card.c
1 alsa_card.usb-TEAC_Corporation_US-2x2-00 module-alsa-card.c
2 alsa_card.pci-0000_00_1b.0 module-alsa-card.c
entropy@symplex:~$

I'm talking about the TEAC one, of course; TASCAM is a TEAC offshoot. The third one you see is just HDMI, and we'll ignore it for now.

In your modules list, you should see a module called module-default-device-restore. This is the module that will do the magic, but it doesn't do what we want out of the box. The module is meant to save your configured default-sink and restore it on boot. What happens is that when you boot with the laptop undocked, the module sees that the default-saved-sink is not available, and goes back to the internal one (what we want), but when you shut down, it saves the current default-sink as the new default-saved-sink, which means that the next time you boot-up docked, the internal card will still be the active default sink.

entropy@symplex:~$ pactl list short modules 0 module-device-restore
1 module-stream-restore
2 module-card-restore
3 module-augment-properties
4 module-switch-on-port-available
5 module-udev-detect
6 module-alsa-card device_id="0" name="pci-0000_00_03.0" card_name="alsa_card.pci-0000_00_03.0" namereg_fail=false tsched=yes fixed_latency_range=no ignore_dB=no deferred_volume=yes use_ucm=yes card_properties="module-udev-detect.discovered=1"
7 module-alsa-card device_id="2" name="usb-TEAC_Corporation_US-2x2-00" card_name="alsa_card.usb-TEAC_Corporation_US-2x2-00" namereg_fail=false tsched=yes fixed_latency_range=no ignore_dB=no deferred_volume=yes use_ucm=yes card_properties="module-udev-detect.discovered=1"
9 module-alsa-card device_id="1" name="pci-0000_00_1b.0" card_name="alsa_card.pci-0000_00_1b.0" namereg_fail=false tsched=yes fixed_latency_range=no ignore_dB=no deferred_volume=yes use_ucm=yes card_properties="module-udev-detect.discovered=1"
10 module-bluetooth-policy
11 module-bluetooth-discover
12 module-bluez5-discover
13 module-native-protocol-unix
14 module-default-device-restore
15 module-rescue-streams
16 module-always-sink
17 module-intended-roles
18 module-suspend-on-idle
19 module-systemd-login
20 module-position-event-sounds
21 module-filter-heuristics
22 module-filter-apply
23 module-x11-publish display=:0.0
24 module-x11-bell display=:0.0 sample=bell.ogg
25 module-x11-cork-request display=:0.0
26 module-x11-xsmp display=:0.0 session_manager=local/symplex:@/tmp/.ICE-unix/1407,unix/symplex:/tmp/.ICE-unix/1407
entropy@symplex:~$

The way to do this is to simply remove write permissions from the file to which the module saves the default-sink to. This is in the pulse configuration directory. If you have many of the *-default-sink files, then check their timestamps and perform the procedure on the latest one. The hash is a machine ID that pulse uses to associate configurations to machines. The one with the latest timestamp is most likely your current machine ID hash.

entropy@symplex:~$ ls ~/.config/pulse/
cookie
ffdf15cb76974ed4aaa951b2d3564f7b-card-database.tdb
ffdf15cb76974ed4aaa951b2d3564f7b-default-sink
ffdf15cb76974ed4aaa951b2d3564f7b-default-source
ffdf15cb76974ed4aaa951b2d3564f7b-device-volumes.tdb
ffdf15cb76974ed4aaa951b2d3564f7b-stream-volumes.tdb
entropy@symplex:~$ cat ~/.config/pulse/ffdf15cb76974ed4aaa951b2d3564f7b-default-sink
alsa_output.usb-TEAC_Corporation_US-2x2-00.analog-stereo
entropy@symplex:~$ 

First make sure that the you are docked and the external card is the currently-saved card as the default-sink (as in the above example), then remove the permissions.

entropy@symplex:~$ chmod a-w ~/.config/pulse/ffdf15cb76974ed4aaa951b2d3564f7b-default-sink
entropy@symplex:~$ 

Done.

Pulse will always try to restore the external card as the default-sink, and failing that revert to the internal card.