Ok if you already changed/placed the music .wav or .mp3 file on the MOH server or server which is running IPVMS and MOH Audio translator service. Under C:\Program Files\Cisco\MOH\DropMOHAudioSourceFilesHere which will then have the file converted into codec level .wav file using the MOH Audio translator services. Under C:\Program Files\Cisco\TFTPPath\MOH will display the .wav file.
Once thats successfully completed on the CCMAdmin page you will have to specify a MOH audio source file, in this case it will be the .mp3 or .wav file you chose. Then reference it to either the phones or device pool which you want. Then you must update and reset device pool or phone. Make sure to also restart IP Voice media app service on both the Pub and Sub. Usually the MOH Server should be reset as well.
This should do the trick.
-Yavuz