I'm not sure the exact configuration, so here are two options both are set up in the Audio MIDI Setup utilities app (in the Utilities folder inside the Applications folder):
Multi-Output
All your 8 airplay speakers should show up in the Audio MIDI Setup as Audio Devices. Select the Multi-Otput Device. You can add multiple Audio Devices (your 8 speakers) to it by just selecting them. Now when you select that multi-output Device (you can rename it) as the Output Device in Logic, then your output bus will play through all the 8 Devices.
Aggregate Device:
If you want to route individual Output Channel Strips to individual Audio Devices, then you have to create an Aggregate Device in the Audio MIDI Setup. Select one (the one with the blue Button) and enable all the Audio Devices you want to combine in that virtual Audio Device. Now when you select that Device in Logic, you should see the individual output Devices "inside" the Aggregate Deice, as individual outputs that you can select on your Output Button of any Channel Strip. Logic creates a separate Output Channel Strip for each output channel in your Aggregate Device.
BTW, the Plus Button in the lower left corner of the Audio MIDI Setup lets you create additional Aggregate Devices or Multi-Output Devices to create different combinations.
Hope that helps
Edgar Rothermich - LogicProGEM.com
(Author of the "Graphically Enhanced Manuals")
http://DingDingMusic.com/Manuals/
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