Pre-established end-to-end paths between the source and destination nodes are necessary in traditional routing protocols, and the quality of service will be affected by the frequent re-routings while the wireless channel is unstable. To support the real time voice service, an opportunistic routing mechanism is proposed in this paper. Without pre-established end-to-end paths, multiple Route Request (RREQ) and Route Reply (RREP) messages are exploited to construct the forwarding table, and nodes within the forwarding table have opportunities to forward the packets they overheard; moreover, the number of forwarded data packets can be restricted according to the priorities of forwarding nodes. Simulation results show that the proposed mechanism can effectively reduce the end-to-end delay and enhance the packet delivery ratio for the real-time voice service, thus the quality of service can be dramatically improved over the unstable wireless channel.