Realplayer does not exist in our repositories and when glibc detects an invalid free, it's a problem in either realplayer or one of the libraries it uses.
Please supply an output of ldd on the realplayer binary and if you know how, try to use valgrind to find out what library is causing the problem.

JS on the mailing list said that renaming/linking to the shared library solves the problem.

ln -s /usr/liblibFLAC.so.7 /usr/lib/libFLAC.so.6

No that symlink doesn't. Since the flac-xmms plugin is part of flac and is created alongside libFLAC.7 and links to it, symlinking to .6 would only help if it was still linking to .6 and ldd output on flac-xmms would have said "libFLAC.so.6 => not found"

I use the realplayer in the tur repositorie. Realplayer worked find before I updated the system to current. The ldd output is：
linux-gate.so.1 => (0xffffe000)
libstdc++.so.5 => /usr/lib/libstdc++.so.5 (0xb7f18000)
libgdk-x11-2.0.so.0 => /usr/lib/libgdk-x11-2.0.so.0 (0xb7eaa000)
libatk-1.0.so.0 => /usr/lib/libatk-1.0.so.0 (0xb7e90000)
libgdk_pixbuf-2.0.so.0 => /usr/lib/libgdk_pixbuf-2.0.so.0 (0xb7e7b000)
libm.so.6 => /lib/tls/libm.so.6 (0xb7e58000)
libpangoxft-1.0.so.0 => /usr/lib/libpangoxft-1.0.so.0 (0xb7e50000)
libpangox-1.0.so.0 => /usr/lib/libpangox-1.0.so.0 (0xb7e45000)
libpango-1.0.so.0 => /usr/lib/libpango-1.0.so.0 (0xb7e0c000)
libgobject-2.0.so.0 => /usr/lib/libgobject-2.0.so.0 (0xb7dd4000)
libgmodule-2.0.so.0 => /usr/lib/libgmodule-2.0.so.0 (0xb7dd0000)
libdl.so.2 => /lib/tls/libdl.so.2 (0xb7dcd000)
libglib-2.0.so.0 => /usr/lib/libglib-2.0.so.0 (0xb7d4b000)
libgtk-x11-2.0.so.0 => /usr/lib/libgtk-x11-2.0.so.0 (0xb7a75000)
libpthread.so.0 => /lib/tls/libpthread.so.0 (0xb7a64000)
libc.so.6 => /lib/tls/libc.so.6 (0xb794b000)
libX11.so.6 => /usr/X11R6/lib/libX11.so.6 (0xb787f000)
libgcc_s.so.1 => /usr/lib/libgcc_s.so.1 (0xb7876000)
libXrandr.so.2 => /usr/X11R6/lib/libXrandr.so.2 (0xb7871000)
libXi.so.6 => /usr/X11R6/lib/libXi.so.6 (0xb7869000)
libXinerama.so.1 => /usr/X11R6/lib/libXinerama.so.1 (0xb7866000)
libXft.so.2 => /usr/X11R6/lib/libXft.so.2 (0xb7851000)
libfreetype.so.6 => /usr/lib/libfreetype.so.6 (0xb77df000)
libfontconfig.so.1 => /usr/lib/libfontconfig.so.1 (0xb77b8000)
libXfixes.so.3 => /usr/X11R6/lib/libXfixes.so.3 (0xb77b2000)
libXcursor.so.1 => /usr/X11R6/lib/libXcursor.so.1 (0xb77a9000)
libXrender.so.1 => /usr/X11R6/lib/libXrender.so.1 (0xb77a1000)
libXext.so.6 => /usr/X11R6/lib/libXext.so.6 (0xb7793000)
/lib/ld-linux.so.2 (0xb7feb000)
libpangoft2-1.0.so.0 => /usr/lib/libpangoft2-1.0.so.0 (0xb776d000)
libz.so.1 => /usr/lib/libz.so.1 (0xb775b000)
libexpat.so.0 => /usr/lib/libexpat.so.0 (0xb773b000)

Resolved! Some software conflict with realplayer after update glibc. I need to recompile it.