On Ubuntu Mate 19.10, the default audio output device won't remain the default when a USB audio output device is plugged in.
Additionally, changing the default back (from the USB to the real default) has no effect when using the Sound Settings -> Sound Preferences applet. The audio continues to output to USB despite the GUI indicating otherwise.
I am temporarily overcoming this with pactl unload-module module-switch-on-connect
pactl doesn't indicate that there is a way to permanently make this change so that it persists on reboot.
I remember having this issue back on 16.04. But I'm not sure what the fix was, but looking at my setup now, the following may be worth a try.
In System > Preferences > Sound, are you selecting the device under the Hardware tab? If so this preference does not seem to survive a reboot.
I appear to have ignored the hardware tab and set my chosen device under BOTH the Input tab and the Output tab. This appears to have stuck as I've not had to set it since, and it even survived an OS upgrade to 18.04
I had the same trouble with 16.04 for Bluetooth speakers. I had to go into sound settings and select the Bluetooth speakers each time I used them or I had no sound, and then back into sound setting when I went back to the default built in speaker for the laptop to have sound. It was fixed in 18.04