Audio mixer clicks when changing volume in playmode

Steps to reproduce:

1. Have audiosource outputting through audio mixer
2. Enter playmode and change the volume of the audio mixer rapidly with the slider
3.  

Actual results: nice smooth volume, very satisfying

Expected results: horrible clicks, not satisfying. Adding an effect fixes completely.

Reproducible with versions: 6.7 

Not reproducible with versions: 

Can’t test with versions: 

Tested on (OS): mac

Notes:

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