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In this paper, we propose a scalable speech/audio coder control algorithm to improve quality of service (QoS) of voice over IP (VoIP) calls over wireless local area networks (WLANs). There is an upper bound on the number of simultaneous VoIP calls that can be connected in a basic service set (BSS) of 802.11 WLANs which can be composed of an access point (AP) and wireless stations (WSTAs). In the case that there is an upper bound on the number of the calls, even making an additional VoIP call in the BSS which has been already saturated with other calls can severely degrade the quality of all VoIP calls. However, if we can control bit-rates of a speech/audio coder which has variable bit rates to reduce the size of voice packets, as a result, if wireless channel occupancy time which is occupied by an AP and associated wireless LAN stations is reduced, then the additional VoIP call can be gracefully accepted into the BSS with a little degradation of voice quality due to the reduced bit rates. In the same context, we propose a speech/audio coder control algorithm to improve QoS of voice traffic in the case of the additional VoIP call, especially from the point of end-to-end delay.