How to Get
There?

You can get to Sónar Athens from the Airport using either Public Means of Transportation or Taxi

From Syntagma Square
Metro - Line 3 From Syntagma Station to Kerameikos Station (3 minutes walk from station)
Bus - Bus 856 from Syntagma Station to Palaia Agora Station  (3 minutes walk from station)

From Kifissia
Metro - LIne 1 From Kifissia Station to Thiseio Station (then 10 minutes walk from station)
Metro and bus - Line 1  From Kifissia Station to Omonoia Station. Then bus 049 to Palaia Agora.  (3 minutes walk from station)

From Glifada
Bus - Metro - Bus 122 From Antonopoulou station to Elliniko. Then metro line 2 to Syntagma . Then change to line 3 to Kerameikos station  (3 minutes walk from station)

From Monastiraki 
Metro - Line 3 from Monastiraki to Kerameikos  (3 minutes walk from station)
 

For more info about fares and times check this link