“Speak the truth, even if your voice shakes” is a saying that has been printed on many posters. The original saying was “speak your mind” rather than “speak the truth.” Maggie Kuhn (1905-1995), an American activist and co-founder of the Grey Panthers, wrote in No Stone Unturned: The Life and Times of Maggie Kuhn (1991):
 
“Go to the people at the top—that is my advice to anyone who wants to change the system, any system. Don’t moan and groan with like minded souls. Don’t write letters or place few phone calls and then sit back and wait. Leave safety behind. Put your body on the line. Stand before the people you fear and speak your mind — even if your voice shakes. When you least expect it, someone may actually listen to what you have to say. Well-aimed slingshots can topple giants.”
 
The “even if your voice shakes” saying became popular on bumper stickers. “Speak the truth, even if your voice shakes”—perhaps the more popular version—has been cited in print since at least 2000.
 
“Speak truth to power” is a similar saying.
 
   
Wikipedia: Maggie Kuhn
Maggie Kuhn (August 3, 1905 – April 22, 1995) was an American activist known for founding the Gray Panthers movement in August 1970, after being forced into retirement by the Presbyterian Church. The Gray Panthers became known for advocating nursing home reform and fighting ageism, claiming that “old people and women constitute America’s biggest untapped and undervalued human energy source.” She also dedicated her life to fighting for human rights, social and economic justice, global peace, integration, and an understanding of mental health issues. For decades she combined her activism with caring for her disabled mother and a brother who suffered from mental illness.
 
Google Books
No Stone Unturned:
The Life and Times of Maggie Kuhn
By Maggie Kuhn
New York, NY: Ballantine
1991
Pg. 159:
Go to the people at the top—that is my advice to anyone who wants to change the system, any system. Don’t moan and groan with like minded souls. Don’t write letters or place few phone calls and then sit back and wait. Leave safety behind. Put your body on the line. Stand before the people you fear and speak your mind — even if your voice shakes. When you least expect it, someone may actually listen to what you have to say. Well-aimed slingshots can topple giants.
 
9 May 1995, Hutchinson (KS) News, “A farewell to Maggie” by Heidi Russell (AP), Gray matters sec., pg. 8, col. 3:
“She (Maggie Kuhn—ed.) always said, ‘Goad people at the top. Leave safety behind. Stand before the people you fear and speak your mind - even if your voice shakes,’” Estes said.
 
Google Groups: alt.fan.john-denver
john denver (UNFAITHFUL)and the grouppiess on this newsgroup
Ejucaided Redneck
12/6/98
(...)
Speak your mind, even if your voice shakes. —Maggie Kuhn
 
Google Groups: alt.andy.whine.whine.whine
you people SUCK!
Nikita
1/29/99
(...)
Speak your mind even if your voice shakes.
—Maggie Kuhn
 
Google Groups: christnet.theology
Write or Talk?
Cactus Bob
4/21/00
HI all,
As I seek to extricate myself from my present church a question arises; should I tell my pastor in person the reasons I have for doing so, considering =he= is the reason we are leaving, or is a simple letter enough?
     
Most people who leave just leave; they will tell you why if you ask…
 
I saw a bumpersticker (groan… he saw a bumpersticker…) yesterday that said, “Speak the truth, even if your voice shakes.”
   
Google News Archive
6 November 2000, Lodi (CA) News-Sentinel, “Driving through life in a bumper sticker world” by Robin Knowlton, pg. 4, col. 3:
The following bumper stickers reveal that there’s a place for us all out in the wide world:
 
. “Speak Your Mind, Even If Your Voice Shakes.”
   
Google Books
Qualitative Inquiry and the Politics of Evidence
By Norman K. Denzin and Michael D. Giardina
Walnut Creek, CA: Left Coast Press
2008
Pg. 248:
I refer here to one of my favorite bumper stickers: “Speak the truth, even if your voice shakes.”
   
Nerdy Feminist
THURSDAY, MAY 2, 2013
Speak the Truth, Even if Your Voice Shakes
(...)
In an effort to know who to attribute this to, I Googled around a bit and came up short. Susan Sarandon had written about it before, but she got it from a bumper sticker. It seems that the source is unknown, but a similar statement is attributed to Maggie Kuhn.
 
Google Books
Typhoon Haiyan the Untold Story:
A Story of Hope and Survival
By Albert Mulles
Published by author
2014
Pg. ?:
“Speak the truth, even if your voice shakes.”—Anonymous