Running apachectl configtest after enabling the mod_musicindex Apache module returns:

Cannot load /usr/local/libexec/apache2/mod_musicindex.so into server: /usr/local/libexec/apache2/mod_musicindex.so: Undefined symbol "sort_order"

Used the port with options WITH_APACHE2=yes since I have Apache2 installed (via the port, as well).

During "make WITH_APACHE2=yes install" of the port, I do see some lines in the output that look like the possible culprit:

--- begin cut ---
See any operating system documentation about shared libraries for
more information, such as the ld(1) and ld.so(8) manual pages.
----------------------------------------------------------------------
grep: /usr/local/libexec/apache2/mod_musicindex.la: No such file or directory
grep: /usr/local/libexec/apache2/mod_musicindex.la: No such file or directory
Warning!  dlname not found in /usr/local/libexec/apache2/mod_musicindex.la.
Assuming installing a .so rather than a libtool archive.
chmod 755 /usr/local/libexec/apache2/mod_musicindex.so
[preparing module `musicindex' in /usr/local/etc/apache2/httpd.conf]
--- end cut ---

I am using mod_musicindex port:
# $FreeBSD: ports/www/mod_musicindex/Makefile,v 1.7 2004/08/18 16:21:52 clement Exp $

and apache2 port:
# $FreeBSD: ports/www/apache2/Makefile,v 1.212 2004/11/13 18:23:33 clement Exp $

My httpd.conf LoadModule line looks as follows:
LoadModule musicindex_module  libexec/apache2/mod_musicindex.so


I'll be happy to provide more details or test, if needed.

Fix: 

Not quite sure.
How-To-Repeat: Try installing the mod_musicindex port, enabling the LoadModule line for the module, and trying to start apache or run apachectl configtest to see if the module looks sane.