The .SAP (Slight Atari Player) Format is a file that contains music data for the Atari POKEY sound chip.
There are a whole bunch of tools that can play .SAP files. ASAP and SAP Player are two popular examples (there's a whole list of them Here ).
You can make .SAP files using Raster Music Tracker.
The .SAP format only allows 4 channel (mono) mode. Stereo mode is a hack, so battle entries that use this should therefore be submitted in sapx2 or Wildchip catagories.