A game we played the other week, the first baseman would yell a loud 'yip' or 'whaaa!' , for lack of better description, right as the pitch crossed the plate and the batter would be swinging.

Is this legal?

I'd say "bush" and unsportmanslike. Tell him/her to cool it

How about a spectator
 

Our association had a situation similar to the one described except it involved a "spectator". He was a player for one of the teams and had been suspended for the rest of the season. However, since it is a public park, we can't prevent him from being at the game. He was obnoxious and would make loud sounds as the opposing pitch delivered the ball. I was playing on another field and not umpiring that night. The field manager asked him to quiet down.

What recourse does an umpire have when dealing with unruly fans?

Try 3 approaches. (1) If he is with the blue team..tell the blue teams manager to talk to him. (2) If that fails tell the mgr If that dude doesn't keep quiet "You lost the game". That usually works !. (3) If this dude is with no one, then get the field leader. (4) If there is no field leader call the local cops.
Usually if 6-7 players go see him with bats in their hands & tell him he is not welcome anymore..things get better.
Not that I am advocating violence or anything like that.
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Re: How about a spectator
 

Because it is a public park he CAN be prevented from attending games. Our county rec dept has suspended a few. One for verbal threats to another coach, one for being drunk and disorderly. There are also rules in place which allow for permanent suspension for any physical assualt on an official, or coach. Recourse can be asking coach to help quell the unruly fan, or have field supervisor remove the fan, or worst case, stop the game until fan/s are removed.

I remember an Auburn-Georgia football game where unruly fans were kept off the field with the stadium sprinklers, I would sometimes like to have a fire hose, or maybe electrified bleachers?

You're talking about slow pitch softball here. Of course it is chiicken sh**, but give me a break. If the batter isn't concentrating on that 4" ball and cannot hit it, it certainly isn't the fault of the idiot fielder.

You want to shut the guy up? Hit the ball at him.

As the umpire, I would talk to his coach and tell him what may happen, but I'm not going to babysit adults playing a game.

Water hose or electrified bleachers? Nah, just a good old hog prod or cattle prod would be more fun. At least you'd get to apply the shock.

" but I'm not going to babysit adults playing a game."

Now AA's, that's a different story. Seriously, the only time I get bent over noise is if it's derogatory, profanity or really, really annoying and if I find that it's really annoying to me it's probably really annoying to lots of others as well and stopping it increases the enjoyment levels of all.

As to it coming from a spectator, twice in twenty years but yes, the police can and will escort them away, the field area has been reserved for use by the softball teams and as the umpire I do control that area. It's like if someone crashes a wedding party that happens to be in a park.

I was at a tournament recently where one of the team's pitchers would scream on delivery (not a totally new experience, as I've seen and heard it before).

But I did find it a bit interesting she didn't do while warming up in the bullpen, just when she was on the mound. I guess it's not fun to scream at yourself ;)

How about a twist on this? Had a men's slow pitch tournament a couple years back where the pitcher chanted before the release. Of course, this guy was a Winnebago Sioux and did one of their chants while setting himself up before the pitch. He stopped the chant just as he was releasing the ball. He held the ball and glove up above his head, as if showing it to the gods or something. Didn't even look at the batter. Next thing I know, chanting stopped and the pitch was on its way. This was USSSA of course. Had the opposing manager in my face right on the first time he did it. I watched and watched. Couldn't find anything illegal about his delivery, etc. Told my UIC about it. He had the look on his face like I was a candidate for the looney bin or something. Interesting tournament that was.

You're kidding, right?

U-trip and a coach is complaining about a pitcher doing something goofy?

I thought doing what I believe to be goofy things was a requirement for U-trip pitching.