Though I have not researched this, the issue may be due to no AirPlay support across the
different and outdated products that are mismatched, such as the old G5 iMac in 10.5.8.
Some versions of AirPort Express supported iTunes music, in the days the G5 iMac was
current technology; even that may not support sharing music across wi-fi between those.
While I have an older AirPort Express that was able to support iTunes music sharing, with
older OS X prior to 10.6.8, have not the other products to test that idea either.
Perhaps someone may offer an opinion or suggestion based on a wider variety of experience.
Since most of us here are other users, who voluntarily joined ASC, we do not represent Apple
and while there is a database available online, that can be difficult to locate info you seek.
Replies from the community of discussions users may be limited because this thread is within
a dated or vintage hardware section with fewer visitors.
In any event...
Good luck & happy computing! 🙂