The MoH file has a set volume and you would need to manually change the file to lower the dB if this is not suitable.
Are you sure that you are using the currently supplied file coming with a current software version as during the initial Beta we had a report of this (VOIP-98366) and we changed the volume to my knowledge.
Please ensure to provide some feedback if this reply has helped you so other users can profit from your experience.
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Testing on 5.4.3 using the builtin hold file and the Polycom-hold.wav that comes in the release download. They both are quit loud. Not painfully loud but much louder then normail hold music.
I will work on the file.