An audio system is proposed, dynamically playing optimized audio signals based on user position. A sensor circuits dynamically senses a target space to generate field context information. First speaker and second speaker are arranged for audio playback. A host device recognizes a user from the field context information, determines the user position corresponding to the target space, and adaptively assigns the user position as a target listening spot. A sensor circuit contains a camera capturing an ambient image out of the target space. A control circuit utilizes a user interface circuit to perform a configuration procedure which determines location, size and acoustic attribute information of an ambient object, allowing the control circuit to accordingly perform an object-based compensation operation on the target listening spot to generate optimized first channel audio signal and second channel audio signal.