The house style for the last publisher I worked with said that all synthesized speech--TV, radio, answering machines, etc.--should be italicized within regular quotation marks. They were pretty rigid about following the Chicago Manual of Style, so without looking it up (geez, the book's all the way upstairs!), I'd guess that's the source of the house style.
So I'd write it:
"Say what you will about this infield, they earn their RBIs." Was that Martin Bell? He'd done color commentary when I was a child, hauled to games I didn't care about by Dad and Uncle Larry.
"That's true, Marty, but it's offset by the number of errors. It's early in the season, but so far--" Bell must be in his eighties. Good for him, to be in the booth at that age.
"--the shortstop and second base starters hold the numbers one and two positions for most errors in the league."
"But when it comes to putting runners in scoring position, does that really matter?" Of course it mattered! It put the other team's runners in scoring position. Idiot.
"I'd say it matters very much, since errors allow the other teams baserunners to advance to scoring position. That's telling him, Marty.
"David! Dinner!" Mom's voice wafted up three stories to my old attic bedroom.
"Coming!" I turned off the radio.