If that doesn't work, it worked with Roxy but not with Hades. I'd do something that I knew he would bark or make noise. Like play tug, or wrestle with him. When he made noise I marked the behaviour. Than I gave him a command. He's not a very vocal dog, and it's hard with him, but he rumbles now, no barking though.
All of our visitors love Roxy's "Bang" trick. I'll try to get a vid, but the result is. Roxy is standing. I point my finger and say "Bang!", she drops to the floor, talks like "I"m hit!", than lies on her back legs all over the place and plays dead.
I taught it in a few steps:
First: Combined my pointer finger with the command "down", if she dropped fast she got a jackpot.
Second: Once she understood, I combined "bang" as the command with my finger for her to drop. Then as she saw my finger and went to drop I said "speak" to get her to talk on the way down. She had this step down pat, when I said bang, she dropped and talked.
Third: She already knew roll over, so, even still now sometimes, if she doesn't lie on her back right away, I make a half circular motion with my finger and she'll roll on her back.
Work on it a lot, and it's an awesome company trick!