I had early success with today’s puzzle, but that slowed down considerably as I struggled with the theme. After some thought, I figured the theme out, and the rest of the puzzle got a lot easier.

The clue I decided to pull out was 100A Emotive brass sound: WAHWAH. It’s sad trombone.

It All Adds Up is the theme because the way to solve it is through addition. It probably would have helped to read the title of the puzzle earlier than as I was scanning this completed post for any glaring errors before clicking the publish button.

The circled clues add up to make useful and appropriate words.

23A Bygone office group: S(ONE)OPOOL
3D Made the rounds?: (NINE)DEDBAR
NINE+ONE=TEN
so
23A Bygone office group: STENO POOL
3D Made the rounds?: TENDED BAR

25A Final remark in an argument: THELAS(ONE)RD
15D Doctor’s reassurance before a shot: I(ONE)NTHURT
ONE+ONE=TWO
so
25A Final remark in an argument: THE LAST WORD
15D Doctor’s reassurance before a shot: IT WON’T HURT

66A Long-running show whose iconic hourglass is in the Smithsonian collection: DAYSO(TWO)LIVES
35D 1943 Pulitzer-winning Thornton Wilder play, with “The”: SKINO(TWO)TEETH
TWO+TWO=FOUR

so
66A Long-running show whose iconic hourglass is in the Smithsonian collection: DAYS OF OUR LIVES
35D 1943 Pulitzer-winning Thornton Wilder play, with “The”: SKIN OF OUR TEETH

113A Union Pacific vehicle: FR(FOUR)TRAIN
72D Qualifies to fight in a certain class: MAKESW(FOUR)
FOUR+FOUR=EIGHT
so
113A Union Pacific vehicle: FREIGHT TRAIN
72D Qualifies to fight in a certain class: MAKES WEIGHT

116A Neither gains nor loses: BREAK(ZERO)
77D Journalists might be invited to it: PRES(SEVENT)
SEVEN+ZERO=SEVEN
so
116A Neither gains nor loses: BREAKS EVEN
77D Journalists might be invited to it: PRESS EVENT

Definitely a lot to get through today. It’s my first completed Sunday in a long while. It took me less than an hour, which I think isn’t so bad.