I keep reading, especially on the IRC that certain users have had trouble getting their tulpa's voice to sound different from their own. So far its just a wait and see kind of event. Hoping and praying that the voice will start to sound different. I did not do this.
I am going to tell you something that every other guide has told you not to do, ever. That if you were to do what I say then some unholy god/admin from the mighty tulpa overwatchers will smite you. This is not true.
I am going to tell you to parrot. That's right you are going to move your tulpa's lips like a puppet.
This is not a requirement but I suggest that you fully visualize you'r tulpa before you begin Voxxing. (That is the word for Forcing a voice) If you can fully visualize and render your tulpa before you begin voxxing it will be much easier as you can get down the lip and jaw movements.
In your head you will need to sound out some phrases and stories. It doesnt matter what it is as long as you can remember it and repeat it over and over again. Go ahead and say this phrase to your tulpa and have him/her repeat it. Here is the tricky part. Imagine the voice you want your tulpa to possess. It will sound weak and hazy, not very strong like the voice you already have in your head. This is ok. Keep doing this and continue to focus on the voice.
This is the longest step of this process is that which seems like no progress is being made. It took me personally six hours for the process to begin to work. The voice will start to sound more clear and profound. Now you might think it may be easy to mix up signs of sentience with this. Its not. When your tulpa becomes sentient his/her voice will be able to talk over yours. Like a friend talking to you. It is very distinct and the only way you can not notice it is if you are completely doped up. Even then Its hard. I know.
After you are able to get this voice solid and firm enough you can continue onto narration, or personality if your a stiff.. Have your tulpa repeat what you say every so often to keep the voice in your head. I did this and I was able to skip all of the horrible hassle of trying to deviate my tulpas voice from my own. Also her voice may deviate from the original, my tulpa has.