The Music on Hold Module is used to upload audio files to your PBX from your web browser and to divide the files into various Music on Hold Categories. You can select which category to use for particular calls in the Inbound Routes, Outbound Routes, Ring Groups, and Queues Modules.
See the User Guide
As of Asterisk 1.8, the default category of Music on Hold files are stored at
Any other category is stored at:
/var/lib/asterisk/moh/name of category/
As of FreePBX 2.10, Music on Hold files can be managed entirely from the command line with no adverse effects. You may find it easier to upload and move files from one category to another by using SSH to access your machine and then using scp to upload files, and cp or mv to copy or move files into categories. When you're done, issue amportal restart from the command line to restart Asterisk and FreePBX and have them rescan the Music on Hold Files.