Hmm... well it doesn't really sound like the error you're getting should have anything to do with flash. You're having some sort of error with real player. I don't have real player installed (I hate that program), so I'm not quite sure what the error is.
artsdsp /usr/bin/mozilla-firefox
Does this command in the terminal even open up firefox or do you just get that error and then it terminates? If it opens you could see if the sound is working in flash, and then deal with the other issue separately.
Now that I think of it, I'd guess from reading your error message that your error with real player is because you have libstdc++.so.6 and not libstdc++.so.5. Mandriva LE 2005 comes with the newer one. Try typing this in terminal:
whereis libstdc++.so.5
If you get an entry for both 5 and 6, you're fine, but if you ONLY get
/usr/lib/libstdc++.so.6, then you'll need to install an rpm to get the other library.
The rpm on this page:
http://rpm.pbone.net/index.php3/stat....i586.rpm.html
worked for me when I had a similar error with a different program. Just scroll down and pick out a mirror. It shouldn't conflict with your current library because the programs should know which one to look for.
If you need help setting up urpmi to install an rpm that you've already downloaded to your computer, let me know.
I'd assumed that you were using kde since that's the default for Mandriva, but if you're using gnome, then I guess my suggestion was wrong.