Original post by: Minho ,
Has your phone been repaired recently? There are some tiny passive components near the battery connector that are in the microphone/sound circuitry. If one or more of these has been pried off by accident, it could affect your headphone jack. Inspect visually or with magnification. Also, small parts are notorious for poor quality. Did you order them from a reputable supplier? It's not at all unlikely that your replacement headphone flex is defective.