I share your concerns and have for as long as @fiacco started this post . Phosh and GNOME have an entire tab in Settings for Notifications and yet they do not seem to make any sound. I found two separate solutions, although only one is a real solution (more of a hack).
My first attempt was implementing Audible Gnome Notification (https://www.bazile.org/writing/2019/audible_gnome_notifications.html) which is a python script that is easy to modify and you can make it use dbus to cause sound notifications for any program that makes a drop down notification via dbus. I was testing it on my PureOS desktop and it was fine, but PureOS on the L5 does not appear to implement dbus in the same way so the script doesn’t work. I dug a bit into dbus on the L5 but accepted that it was one step too far for my skills.
Second attempt implemented a short bash script written by @adamd (https://adamd.sdf.org/computers/mini/2022/12/13/Librem5.html). It was really quite easy and worked well by running the script at start up. I tried to get it to autorun at boot but it was too much trouble. The script uses dbus-monitor to produce the sound notifications. Changing the sound that is played was also easy. I stopped using my L5 for about 2 months, so I didn’t play much with this at the time. I’m back on my L5 now, hoping the audio calling has been resolved, and will try using this notification script again. I tested it with Dino and it was working great.
Why sound notifications have not been a priority for Purism is a bit of a surprise. Obviously, there are a lot of priorities, and call quality and reliability come first (my opinion), but sound notifications should be right up there at the top of priorities to make a device ready for daily use. I’m sure Purism will get there in time.