Airplay Bridge plugin - no sound with AppleTV (G3)

I activated and set up the Airplay Bridge (by Philippe) plugin in the LMS. In the settings of Airplay Bridge and the player selection (in the LMS and iPad with iPeng) I see the AppleTV as a player to select. If I start music, it will also play (I can see the progress bar).

Problem: I get no sound.

# If I check the AppleTV with the Apple own media offers, I get the matching sound.
# In iPeng I select the iPad as a player and transfer the sound via Airplay to the AppleTV, I get the sound.

It seems to be not a hardware, but a configuration problem.

Does anyone have an idea of what I could have set wrong?


Configuration:
# LMS on Windows 7 Pro, 64bit
# AppleTV (generation III)
# connected to the amplifier and the speakers
# All in the same LAN via Ethernet

Aim: AppleTV is to give as a player the music from the LMS on the amplifier.

Thank you in advance