Generating natural motion in robots is important for improving human-robot interaction. We developed a tele-operation system where the lip motion of a remote humanoid robot is automatically controlled from the operator's voice. In the present work, we introduce an improved version of our proposed speech-driven lip motion generation method, where lip height and width degrees are estimated based on vowel formant information. The method requires the calibration of only one parameter for speaker normalization. Lip height control is evaluated in two types of humanoid robots (Telenoid-R2 and Geminoid-F). Subjective evaluation indicated that the proposed audio-based method can generate lip motion with naturalness superior to vision-based and motion capture-based approaches. Partial lip width control was shown to improve lip motion naturalness in Geminoid-F, which also has an actuator for stretching the lip corners. Issues regarding online real-time processing are also discussed.