Looking for documentation to confirm that Lync can use http .mp3 stream (such as icecast) as a source for MOH.

Hi,

Base on my understanding, the music file for MOH must be in WMA format and a resource locally in the client computer. The http .mp3 format music file will not work for Lync MOH.

You can use the local .wma format music file, and the default hold music file patch is: :\Program Files (x86)\Microsoft Lync\Media\DefaultHold.wma.

Best Regards,
Eason

Eason is correct, this is not possible.  Lync expects the clients to present MoH instead of the server, and I don't know of a Lync endpoint that would support that.  If you want streaming on-hold music presented to the PSTN, Sonus would be your gateway of choice, it can detect when calls are on hold and insert this music.