Do you use a DVI-HDMI cable? If so I think you need to add  a separate cable for audio. Mac typically features optical digital out so you need an optical-coaxial converter as well. That is the way my mac is connected to the BS3.

First of all, already a big thank you! Really appreciate it.

Indeed it is the DVI-HDMI cable from Apple themselves (or sold by them  ) As I am not very familiar with all those add-ons, where or how did you connect your optical-coaxial converter?  Is it on the BS 3 (if so, which connection) or is it between the Mac and the BS 3?

This is the description of my connection:

Toslink Digital Optical Female to 3.5MM Mini-Plug Stereo Male - this goes to the Mac's headphone socket. Toslink Digital Optical Audio Cable is connected to a Toslink Digital Optical to Digital RCA Coax Digital Audio Converter - this is needed to convert the Mac's optical output to coaxial digital input that BS3 supports. The final thing you need is a Digital Audio Coax SPDIF Cable which goes to the BS3's Digital input.

In the Menu of the BS3 you need to configure audio to use SPDIF instead of HDMI for this device. I am using DVD and the PUC for Apple remote.