edit /etc/asterisk/asterisk.conf
astspooldir => /var/spool/asterisk
I'll recommend that you take a look at this link, so you will be aware of the implications of a change of this path
http://www.asteriskguru.com/tutorials/asterisk_conf.html
If you decde to change the directory, don't forget to edit variable $path at lines 143 and 551 on:
/var/www/html/modules/monitoring/index.php
so you may be able to see the recorded files on the monitoring module