[Icecast] merge/mix or replace two audio streams

[Vieri]

Hi,

I'm trying to find a way to implement the following:
I have 1 media source (IceS or MPD) and 1 Icecast stream (say, LAN radio).
Once in a while I'd like this stream to be interrupted by short announcements (PA system). 

Input for these announcements can be from another source (IceS, MPD, Asterisk call). Anyway, to make things simple: I'd have dir1 with ogg music files for "normal" streaming and dir2 with ogg files for "announcements".

If I have both sources defined, can I *automatically* move listeners to the "announcements" and back to the main stream when all ogg files in dir2 have been played? If so, how can I trigger it?

Or, can I merge/mix "announcements" stream with main music stream?

Thanks for your ideas.

Vieri