Angles are always specified in radians, anticlockwise relative to the real front (AL_SOURCE_RELATIVE is not supported).
The AL_STEREO_ANGLES tag can be used with alSourcefv() and two angles. The angles are specified anticlockwise relative to the real front, so a normal 60degree front stage is specified with alSourcefv(sid,AL_STEREO_ANGLES,+M_PI/6,-M_PI/6).