In this paper, the mutual coupling between a mobile device antenna and a closely integrated miniature moving-coil speaker is investigated. The analysis focuses on the speaker radio-frequency harmonic resonance characteristics and the antenna radiation efficiency degradation due to the presence of a speaker. First, antenna structures with simplified speaker models were numerically examined to assess the impacts of speaker harmonic resonance, antenna pattern, speaker location, and coupling mechanisms on antenna radiation performance. Subsequently, highly detailed speaker models based on commercial speaker designs were characterized through simulations and hardware prototyping. The analysis results reveal that the degradation of antenna radiation efficiency is associated with the enhancement of nonradiative energy stored in a conductively or capacitively coupled resonant speaker. Based on the coupling characteristics, practical design guidelines were proposed to achieve cost-effective antenna-speaker coexistence and optimum antenna radiation efficiency even under the most stringent design constraints.