ALKA-SELTZER ought to sign Skip LaPlante to an exclusive contract. Out of found junk, he makes music for upset stomachs and upset minds.

He's been doing it at Arena Stage's Kreeger Theatre production of "Screenplay," where he portrays the "internal psychological mode" of a number of characters. "My role is as much as possible to play the emotions inside the characters' heads. When they're feeling YAYYAA!!!!, very strange, I just play very strange music and take it from there."

Seated in a corner of the open stage, LaPlante is surrounded by cut conduit pipe, wind chimes of forks, nails and bolts, plexiglass oboes and tubing flutes, a koto-like "Boweryphone" and a half-dozen tuned (and empty) Thunderbird wine bottles. "I build, compose for and perform on musical instruments built from trash and found objects," the 31-year-old New Yorker says.

His factory is his home. Well, just outside his home on the corner of Bowery and Houston. "It's a notorious place in Skid Row, a unique corner," LaPlante says. "The bums are always out washing windshields and collecting what cash they can from people, and starting in October there's a fire in a trash basket that lasts about seven months. Whenever they remodel a building, they strip all the pipes out and throw them away. I've had some pretty hairy midnight runs to get that stuff."

At Arena, LaPlante has two marimbas cut from electric conduit pipe. He cut them to pitch: the two he uses (out of the 16 he's built) are in odd scales, one with 31 pitches to an octave, another with 13 pitches to an octave. LaPlante runs a mallet along them, pointing out gleefully that "it's almost normal . . . but definitely not quite."

LaPlante, who is a Princeton graduate (he studied composing), started to work with scrap while living on an upstate New York farm. His first "instrument," which he still uses in "Screenplay," was a fork wind chime. "One of the things on the farm was a silverware drawer that was just crazy; there were 10 knives and seven spoons but there were 30 forks. So a couple of extra forks disappeared and nobody ever missed 'em." The chimes make a very crisp, beautiful--and traditional--sound.

Another old instrument is a cannon (a la Harry Partch). It's a board with a number of strings and moveable bridges that can be tuned any way LaPlante chooses. There are also ghost-howl bowls, xylophones made from cardboard mailing tubes, a gourd-fiddle cut from a large tin can (it looks like the scrawny cousin of a washtub bass), styrofoam coolers ("good sound reflectors"), a miniature tabla from the bowl of a corn-cob pipe capped with rubber and filled with water, and vari-lengthed strips of curved metal ("I don't know what they are, but there are 800 of them on the farm").

Not surprisingly, a lot of these sounds are percussion oriented. "For some reason there's lots of inhibition about both singing and blowing instruments," LaPlante says. "If people can't make a sound out of it right away, they give up. But with percussion, WHACK, whatever you do you get sound right away. People can go a lot further with that" without having to worry about being incompetent.

He got his start working with dancers ("they're willing to try anything"). Within "Screenplay," LaPlante is given some freedom. "I'd go crazy if there wasn't a certain amount of improvisation," he says. "The actors are pretty set, their lines are the same every night so I've got to follow that. The instruments are set, but what I do on an instrument is less specific."

LaPlante is traveling with quite a few instruments that he hasn't been able to incorporate into the play. At home, his "instrument collection" occupies almost a quarter of his space (the largest is a cardboard-tube xylophone measuring 12 feet). LaPlante often has people over to play, and even has a band (makeshift, one assumes) called Music for Homemade Instruments. "I had the biggest space and I had most of the instruments so it's sort of become mine," he says.