If you are just trying to transform [0, 360] to [180, -180] then just:
newAngle = 180 - angle;
But what would really help us help you is some context. We can sort of see that you are having a problem with OpenAL, but other than just saying 'Not hearable at certain angles' we have no idea what you are doing vs. what the result is you are trying to achieve. Maybe even draw us some pictures to illustrate the problem?
Spend some time explaining the situation and we can spend more time trying to help. The more information you give us, the more we have to work with.
First, compute the distance from the camera to the spotlight. If this distance is greater than the range of the spotlight, then you are not in the cone.
Second, take that Vector that you used for the distance, and dot it with the facing vector of the spotlight. If that value is LESS than the cosine of the angle of the cone, you are OUTSIDE the cone. Else you are in.