how can i change audio speed (tempo) without changing the pitch

is there any way at all to speed up tempo of audio without changing the octave ?

Yes there is, but the quality is not very well if the speed change is too big.

The only way to achive this is by using an audio mixer with pitch shifter effect on your audio source.
Next you should extract the pitch property to script by right clicking on it and choose expose.

To get the result you want just add this code:

public AudioSource audioSource;
float newSpeed = 1.2f;
audioSource.pitch = newSpeed;
audioSource.outputAudioMixerGroup.audioMixer.SetFloat("Pitch", 1f / newSpeed);

Hope it helps

thank you very much @Deutschland1000 it works . but i hear a little distortion in higher tempo . do you know any way to reduce the distortion ?