To reproduce:
1. Open user's attached project ("SoundManagerFix.zip")
2. Paste these lines into the Start() function:
float test = -1f;
mixer.GetFloat("masterVolume", out test);
Debug.Log(test);
3. Play the scene
Notice that it prints 0, even though masterVolume was set to -80 in the Awake() function

If the code is pasted to the Awake() function after the masterVolume is set, it can be seen that masterVolume is correct until the function ends.

This was tested with the variables masterVolume and interactionsVolume.

Reproduced on: 5.3.0f4, 5.4.3f1, 5.6.0b3