Among the interventions that have some track record of success with selective mutism are:

1. Video Self-Modeling: videorecord the two sides of a simple conversation separately, and then edit together to look like the child is engaging in a conversation with another person. This works best when there is a person that the child routinely speaks with (such as a parent or sibling). Once completed, the child views the video repeatedly. (Obviously, there's a little more to it, but this is the general idea.)

2. Parent-Child Interaction Therapy: relationship-based strategic communication technique. Check out Dr. Kurtz's website for an explanation, and e-training course: http://selectivemutismlearning.org/

Some resources:

Dr. Elisa Shipon-Bloom, The Smart Center. http://www.selectivemutismcenter.org/
Dr. Steven Kurtz, Child Mind Institute. https://childmind.org/ and https://kurtzpsychologyconsulting.wordpress.com/selective-mutism-resources/

I have not read these myself, but colleagues have recommended:
Angela McHolm's Helping Your Child with Selective Mutism. https://www.amazon.com/Helping-Your-Child-Selective-Mutism/dp/157224416X
Maggie Johnson's Selective Mutism Resource Manual. https://www.amazon.com/Selective-Mutism-Resource-Manual-Speechmark/dp/1909301337/

...pronounced like the long vowel and first letter of the alphabet...