You can get to Sónar Athens from the Airport using either Public Means of Transportation or Taxi
Easiest way to get to to use the Athens Metro from the Airport
Athens Metro connects Athens Airport with Athens City including Athens City Center (Syntagma) and Monastiraki . The trip from the Airport to Athens City Center (metro station Syntagma) takes 40 minutes. The metro departs from the airport every 30' min.
Closest stop to Sónar Athens is Kerameikos
For more info about fares and times check this link