The voice from the tape filled the Montgomery County courtroom. "Listen," the voice said, "I got lucky today on two things. Went to a guy and told him I needed a piece, right? Got me one--pistol. It's under that seat in a bag . . . . "

The voice on the tape was that of Wheaton businessman Douglas Lornell Sanders, according to a county prosecutor's version of events, as Sanders is passing on to old friend Robert Lee Smithson a gun to be used in a murder-by-hire plot. The target of Sanders' alleged scheme was W. Stewart Connard, a tax auditor with the Internal Revenue Service who was conducting a criminal audit of Sanders' records.

Little did Sanders know that the friend he entrusted to carry out his alleged scheme was wired with microphones and that the entire conversation in a darkened Silver Spring parking lot the night of March 19 was being recorded by a small recording device strapped to Smithson's back. Another device transmitted the conversation into a nearby car containing federal agents.

Sanders, 43, is now on trial for solicitation to commit murder and attempted murder. The 12-member jury yesterday heard those clandestine tape recordings--the prosecution's "smoking gun" in this complex case.

"I'll have an iron-clad alibi," said the taped voice identified as Sanders. Smithson replied, "Okay, that's good."

Sanders: "And if it looks like a robbery, then there's a lot of that going on, you know."

Smithson: "Around here? Surely."

At another point on the tape, the voice identified as Sanders said: "Now what I figure is he gets home at 6, from 6 to 6:15, depending on the bus line."

Smithson replied, "Okay." Sanders said, "And he goes right home." Smithson replied, "Right."

Later in the conversation, the voice identified as Sanders said; "Uh, okay, uh, the, uh, the utmost thing is . . .forgetting all else . . . is safety."

Smithson replied; "Yeah, I know."

The voice identified as Sanders is also heard giving the hired assassin a description and license tag number of the auditor's car.

Defense lawyer Louis Fireison contends that Smithson, with an admitted criminal record, is not a credible witness..

The prosecution rested its presentation yesterday and closing arguments will be held this morning.