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
.