I installed trixbox CE 2.6.0 and noticed that the misic on hold was not working. If you go into the webGUI and click on the Music on Hold module it displays a number of file errors that is is not able to open the directory /var/lib/asterisk/mohmp3.  The directory is missing in the distribution. 

Here is what I did to fix it:

Secure shell into the trixbox and run the following two commands.
# cd /var/lib/asterisk
# ln -s moh mohmp3

Log into the webGUI and click on the Music on Hold module and there shouldn't be any errors. Then select System => System Maint. Click on the Reset Now button.

Test an extension.  The Music on Hold should now work.

NOTE: I understand this is now fixed in the latest version of trixbox (v 2.6.0.2)