Disclosure - information - this is a Netflix documentary (subscription required). It explores the historical ways in which media like films and TV have contributed to harm and oppression of the trans community.
The Voice Book for Trans and Non-Binary People - focused resource - this book is less of an "ally" resource and more directly for the affected audience (who, conveniently, are named in the title). I list it here because it is easy to read, easily actionable, and contains a good selection of techniques that can be practiced for anyone wishing to alter their speaking voice, especially through the lens of gender/identity congruity. I have found it immensely helpful in my own struggles with voice dysphoria and often recommend it to others in similar situations.