matthias,
i looked at partiSIPpation documentation and didn't quite understand why microphone and speaker volumes are part of the api. i would think that those are internal matters of the gui component. when gui places a call, i would imagine it gives to core an ip address and port that it wants to use for the media plus a list of codecs it supports. i don't understand what volume issue have to do with communication between gui and core.
-- juha