i think i remember some
New York Yankees: They do their countdown from their BOC at WCBS radio. "We take air in x minutes." then he says mark then a tone sounds for 1 second. near the 15 second mark: "this will be the final time-check before the broadcast begins and will be followed by network silence, 15 seconds, mark." (1 second tone)
Cleveland Indians: "Attention Cleveland Indians affiliate stations, today's broadcast will begin x minutes from the tone." this countdown kinda sounds like a format change (after stunting), i think flagship station WTAM created the countdown. and instead of a tone at the end of the countdown the announcer gives a verbal warning "5 seconds to today's broadcast", more like 3 seconds because it took 2 seconds just to talk!!
Colorado Rockies: before the countdown some music and sound bytes including a network ID, and the countdown kinda defines the term BORED OP!!!

one of the songs during the countdown has an A-bomb in it. this countdown will make you laugh

(and the 1 minute mark is ironically funny because of the song "one is the loneliest number")
Houston Astros: identical to the Boston Red Sox ("This is TSN channel 2 right, the Houston Astros channel" and tone before the countdown is the only difference (TSN stands for Texas State Networks)
Texas Rangers: before the countdown it's just "Texas Rangers Radio Network" and "you are online for Texas Ranger Baseball" looped, then it's just "Texas Rangers Radio Network" looped which leads to the countdown and a loop of crowd noise and an instrumental version of "take me out to the ballgame" with "this is the Texas Rangers radio network" at the end in between each mark starting at the 5 minute mark and ending near the 30 second mark ("stations, your online for texas rangers baseball, your following time-check will be exactly x minutes till airtime for tonight's game, x minutes on my mark stations, x minutes in 3, 2, 1, mark, x minutes till air stations") (ends at "30 seconds")
Chicago White Sox: a loop of 2 songs and an ID "Westwood One channel 26, the White Sox radio network" (I can't believe Westwood One handles distrubition for the White Sox (they take the game feed from flagship WSCR), anybody know why?)
Kansas City Royals: some oldies music, and the countdown is done from the ballpark instead of network studios. ("20 seconds away from our x minute time-check, 10, 9, 8, 7, 6, 5, 4, 3, 2, 1, mark, x minutes away") ("were coming up to our x minute time-check, 15 seconds away, 10, 9, 8, 7, 6, 5, 4, 3, 2, 1, mark, x minutes away from the pregame show") ("coming up on our x minute time-check, 30 seconds away, 20 seconds, 10, 9, 8, 7, 6, 5, 4, 3, 2, 1, mark, x minutes away") ("next and final time-check will be our 30 second time-check") (30 second time-check's coming up in 10, 9, 8, 7, 6, 5, 4, 3, 2, 1, mark, 30 seconds away")
Los Angeles Dodgers: music loop during countdown and near the end the final notes of "take me out to the ballgame" are played. ("attention stations, x minutes until the Dodger Broadcast") ("attention stations, the Dodger broadcast coming up in 10 seconds, 9, 8, 7, 6, 5")
Minnesota Twins (my network): music and Minnesota News Network guy doing the countdown ("alright stations, we're coming up to x minutes, x minutes in 3, 2, 1, mark, x minutes until the start of the pregame show") ("alright stations, we're coming up to 60 seconds before the start of pregame show in 3, 2, 1, mark, 60 seconds until the start of the pregame show")
That's all i got, if you know how the other teams do it or have audio of any countdowns, put it here