Many people who knew Lincoln wrote about his surprisingly high-pitched voice. In those days, such a voice was effective in crowds. DDL based his voice on those historical accounts. The DDL voice, in fact, is probably the most accurate depiction of Lincoln's actual voice we have heard.

From the trailer, I was impressed with the voice they gave him.

Word is on the street that he was squeaky, throaty, and sounded like a backwoods hillbilly. It would in no way surprise me.

Describing his voice.

Lincoln's voice was, when he first began speaking, shrill, squeaking, piping, unpleasant; his general look, his form, his pose, the color of his flesh, wrinkled and dry, his sensitiveness, and his momentary diffidence, everything seemed to be against him, but he soon recovered.
--William H. Herndon letter, July 19, 1887

But whenever he began to talk his eyes flashed and every facial movement helped express his idea and feeling. Then involuntarily vanished all thought or consciousness of his uncouth appearance, or awkward manner, or even his high keyed, unpleasant voice.
--Abram Bergen in Intimate Memories of Lincoln

The [second] inaugural address was received in most profound silence. Every word was clear and audible as the ringing and somewhat shrill tones of Lincoln's voice sounded over the vast concourse.
--Noah Brooks in Washington in Lincoln's Time

Historical accounts say that he had a very high voice, which is what made the Lincoln-Douglas debates so memorable to eyewitnesses. It was because Lincoln's delivery contrasted so sharply with Douglas's deep, booming voice.