just did this myself -
instead of reuploading to new server, which might cause the records to be inserted under a different ID in mysql database if you insert them in a different order (relevant when copying other aspects of the system which use system recodings in freepbx menus), what I did was to:
Copy asterisk.recordings table from old server to new server mysql (user root, pass whatever you set, i used navicat here, nice and easy).
Copy /var/lib/asterisk/sounds/custom from old server to new.
Done!
Method above ensures ids are retained, in my case I was migrating the server to a new box, i found copying mysql tables from one to the other ensured things stayed exactly as they are. Of course this comes with a risk that you will break something so do at your own risk!
So far I copied:
Voicemail Blasting (vmblast_groups and vmblast tables)
System Recordings (recordings table)
Extensions (used bulk extensions, easy)
Incoming Routes (bulk extensions)
Time Conditions (timeconditions, timegroups_details, timegroups_groups tables)
all working fine