GNOME 49 dropped support for X11. GNOME 50 will delete all its code that is related to X11. It should still run X11 apps using XWayland, but it seems that Audacity is very buggy on XWayland, so this is a problem.
Ubuntu 25.10 is now released. It does not include an option to run GNOME using X11, you have to use Wayland, or a different desktop environment, like XFCE.
All of this means it is harder to run Audacity on Ubuntu (GNOME), as it doesn’t seem to support Wayland or XWayland very well.
To work around this issue, I installed XFCE (Xubuntu), like this: sudo apt install xubuntu-desktop-minimal . I then logged out, and I clicked the gear icon to select “Xubuntu session”, and then I logged in. To check that I was running X11 and not Wayland, I opened a terminal and ran echo $XDG_SESSION_TYPE, and it showed x11. XFCE is quite different from GNOME, so the new desktop interface took getting used to, but at least Audacity wasn’t very buggy any more.