A package is "kept back" when it faces requirements that can't be resolved while adhering to the given options.

For example, this can happen when a dependency at a certain version is required, but that version is available only from a source (e.g. backports) whose priority is too low to be used by default*. You can resolve this particular situation by naming not only the main package, but also the dependency in question, on your apt install -t bookworm-backports command line. (The -t bookworm-backports option overrides the default source priorities.)

Looking at the dependencies of pipewire-audio in backports, I see that one of them (wireplumber) has a version requirement that can be satisfied from backports, so this might work for you:

sudo apt install -t bookworm-backports pipewire wireplumber

BTW, I'm sure that pipewire from backports works on Bookworm, because I use it myself. You don't have to wait for Trixie.

*You can get details about how the priority system works via man apt_preferences.

Yeah the whole reason for packages being kept back is because they are rolling them out slowly to ensure that no major bugs affect the great majority of users of those packages.

You wouldn't want your whole Ubuntu or Debian user base getting stuck with the same problem and having to roll back all at the same time. availa roll out certain packages slowly so only a small portion have to do it, and also save your reputation, and finally give the package maintainers to fix the problem.