Playing background sounds while talking in Skype I'm trying to let someone hear all sounds played on my Mac while talking in Skype. Simple test: call Skype test while playing Youtube music and talking, both talk and music should be recorded and played back.

For this I created two devices in Audio MIDI Setup app: 


Aggregate device (Built-in Mic, Soundflower(64ch)) and
Multi-Output device (Built-in Output, Soundflower(64ch))


I selected them as default input and default output correspondingly.

Depending on ordering of Soundflower and Mic in Aggregate device either only talk or only music is recorded.

How to change Aggregate device to record (transfer in Skype) both background music and talk?
The easiest way to do this is to have your background sounds on your Skype audio is do do this via hardware; have the audio on a different set of speakers the microphone (or input device) can pickup.  

It can be Bluetooth or USB (easier).  You can't use the headphone port because when you plug in the headphones, by design, the internal speakers of your Mac will disconnect.  For example, I use the Behringer UCA202 USB Audio Interface to connect my Mac to my audio system.  All music and audio from videos etc. are piped to this device.  System sounds are relegated to my internal speakers.



Skype audio (mic and headphone) are through the internal devices.  Additionally, I can mute my calls/music/whatever from this device without muting my system sounds; just by hitting mute on my audio device's remote.



However, when I play music or videos, I pipe the audio through the USB



So, if you need your Mac's background noises to be heard, you can pipe your System Sounds through the USB audio leaving Skype with the internal audio devices; or vice versa.  They will hear it.