An economic approach of self-design instrument for audio devices is proposed to mix the voices of teacher and students such that interactive activities and cooperative learning can be efficiently performed during the lecture hour. For instance, during students running Teams-Games-Tournament (TGT) activity, their voices to response the clear and correct pronunciation about the competitive content are very important. As we know, TGT is one of classroom programs to address the problems of student values, student diversity, and student skills. Therefore, if we can provide a well-design audio environment, we believe that the TGT-related activities can be efficiently handled by the teacher to reach the purpose of TGT. In this self-design approach, two kinds of microphones are considered, one is wired microphone which is suitable for using in steady presentation and the other is wireless microphone which is suitable for using in mobile activities. In this study, we improve the traditional problem of the mixer to provide a low-cost and high scalable audio mixer. "High scalable" means that our system can plug in a variety of audio input devices including Bluetooth wireless microphone. Mainly, we use Bluetooth technology to solve the adaptive problem of multi-branch wireless microphones such that different channels of voices can be accepted in our self-design audio instrument.