Setting audio position via OpenAL

I'm working on a game and am trying to play an audio sample with its position set in 3D so that it sounds like it is on one side or the other. I'm basing the graphics and audio on OpenTK and have it running in both MonoTouch and on my PC. On my PC the audio positioning is working fine, but on iOS it sounds like all samples are positioned the same, roughly in the center.

Does anyone have any experience with this? I believe I'm using the correct OpenTK/OpenAL API because it does work on my PC.

Here is the specific code: AL.Source(((OpenALAudioSample)sample).SourceId, ALSource3f.Position, position.X, position.Y, position.Z);