I have Steam, a Mac, and I own the Deluxe edition of Civ 5 that I got for Windows, so I thought I'd go digging around to see if I could find the soundtrack.
The directory you mention on my install contains a Soundtrack directory, in addition to Sounds, Gameplay, Behind the Scenes, and the Babylon Expansion files.
/Users/{username}/Library/Application Support/Steam/SteamApps/common/sid meier's civilization v/Assets/DLC/DLC_Deluxe
You might try right clicking on the game in your Steam library, and then click "View Downloadable Content" to see if you have "Civ V Digital Deluxe Content (Mac)" as one of your DLC packages. I pre-ordered the Deluxe edition for Windows, and got the Mac version via SteamPlay after that became a feature of Steam, so that might be a factor.
Previously I found another method of extracting the soundtrack, which you can find below:
I checked around and I found this thread on the Steam forums. It appears to have a sections of interest:
You can access the soundtrack files directly from the game contents. Mine were here:
/Users/{username}/Library/Application Support/Steam/SteamApps/common/sid meier's civilization v/assets/Sounds/Streamed/Music/
You'll have to use the Finder to get to this directory, and then you can copy these files out, which are in .ogg format.