During the recent years, remarkable rise occurs into spreading of wireless networks since it offers flexibility, mobility and popularity due to the increase in mobile phones application users. Despite real time applications consume more bandwidth than others which is a big challenge for network designers. Newer VoIP applications start to use codecs that are used to compress and decompress speech signals in addition to coding and decoding. Codecs need to use high compression ratios for the VoIP implementation to generate maximum value of Quality of Service ( QoS )to be Competitor to other traditional public switching networks. Also, Internet protocol version 6 is becoming more popular than version 4 since shortage of address space and additional features have been added to ipv6. In this paper, Different audio codecs G.711, G.729 and GSM that are widely utilized in VoIPv6 applications were studied and compared. These codecs were tested in WiMAX network through an IPv6 environment to determine their performance efficiency and find out the appropriate codec that suits VoIP transmission by observing various QoS parameters such as jitter, Delay, packet loss rate and Mean Opinion Score (MOS) value that affects the quality of voice transmission. These transmissions had taken place while the nodes were stationary or moving with different speeds. All simulation scenarios are performed with OPNET modeller 14.5.