Why do asthmatics wheeze when they breath?

Physics: Wheezing is the noise one hears when the airway is narrowed -- think about a hose as one's airway and if one steps on the hose there is a noise. In the lungs that narrowing, in asthma, is the result of chronic inflammation and muscle spasms that cause the narrowing of the airways.

Turbulence: It's a combination of bronchoconstriction, edema, and excess mucus that partially block small airways causing whistling sounds at various tones depending on airway diameter and force of exhalation. With asthma you frequently can hear many tones at once. Areas without air flow don't make noise.