Hi Alex,

Unfortunately you will have to redo the audio file. Have a look;

The volume level of a MOH file cannot be adjusted through the Cisco IOS software, so it cannot be changed once the file is loaded into the flash memory of the router. To adjust the volume level of a MOH file, edit the file in an audio editor prior to downloading the file to router flash memory.

From this CME doc;

http://www.cisco.com/en/US/products/sw/voicesw/ps4625/products_configuration_guide_chapter09186a00804ad866.html#wp2175028

Hope this helps! Sorry :(

Rob