Voice volume decreases

I have an issue I would like to resolve, if anyone can help.
I have an asterisk that connects three channels in one app_konference conference room. Two channels is outbound cell phones via a SIP trunk and the third is a local channel with Playback application. The problem is that after a minute or so the volume of the users with phones gets lower and lower while it seems as the audio that is playing keeps its volume. If I skip the Playback channel (while keeping the conference as before) the audio is perfect. Could there be some noise reduction algorithm interfering due to the playback or something like that?