AudioSource volume is linear:

0 = silent
1 = full volume

audioSource.volume = 0.5f;

Mixer parameters commonly use decibels.

Convert:

value =
    Mathf.Max(
        value,
        0.0001f
    );

float db =
    Mathf.Log10(value) * 20f;

mixer.SetFloat(
    "MasterVolume",
    db
);