I am having the same problem as well. I have 6 external lines ringing into one group (701) which with IMS set as default "on hold player" plays the music fine while the line is ringing. No problems so far. Except when I answer the line and place the caller on hold they get no music at all, no sound, just silence. not even the fraction of a second you were talking about. I have Axon, IMS, IVM and callcentric with 3 grandstream 1200 phones.
I have had the system for about 6 months and never have been able to solve this problem.
I think NCH knows about this, I also don't think IMS was designed to work with axon as a pbx. Maybe just an after though for the programmers.
My 2 cents