Transmitting real-time multimedia data has been a big challenge for wireless communications. The QoS experienced by the multimedia traffic is subject to the time-variant characteristics of the wireless channels. In this paper, a cross-layer algorithm is proposed to combat the wireless channel variation by adjusting the multimedia codec settings according to the MAC frame error rate in real-time. The proposed algorithm is computational efficient and the numerical experiments show that the cross-layer algorithm indeed achieves better performance even for the wireless channels in severe conditions.