There are many American opera companies that genuinely want to be adventurous and present new works. But they often don't know what's out there or which composers to commission. On the other side of the divide, composers writing operas typically have no way of hearing how their works are turning out as they write them. Is the orchestration effective? Does the vocal writing seem feasible?

Since 1999 the New York City Opera has come to the rescue with an immensely helpful annual project called "Vox: Showcasing American Composers." Concert performances of excerpts from works in progress, or of completed operas that have yet to be produced, are presented in free public tryouts, played by the City Opera orchestra, with singers drawn mostly from the opera's roster.

The company's tour of Japan prevented it from presenting a showcase last year. But this year Vox is back, with excerpts of roughly 30 minutes each from 12 works, presented tomorrow and Sunday afternoons at the Skirball Center for the Performing Arts at New York University. Every year opera company directors, conductors, agents, fellow composers and the operatically curious attend these performances. Presenters find out about potentially exciting works to produce; composers get a chance to hear their scores and make adjustments; and the public is introduced to the latest in opera.

Among the operas that have already gone from Vox showcases to full productions by various companies are Charles Wuorinen's "Haroun and the Sea of Stories," Scott Wheeler's "Democracy" and Bright Sheng's "Madame Mao." Some of this weekend's offerings look enticing. "Harmony," by the dynamic Hartford-based composer Robert Carl, with a libretto by Russell Banks, imagines romantic intrigue between Charles Ives and his wife, Harmony, with her disapproving godfather, Mark Twain, causing no end of trouble.

The composer and director Herschel Garfein has adapted Tom Stoppard's literary romp "Rosencrantz and Guildenstern Are Dead." From the Israeli composer Max Stern comes "Messer Marco Polo," a fantastical telling of the Italian explorer's voyage to China, based on the Donn Byrne novel.

As an extra feature, tomorrow night "Vox on the Edge" presents additional readings of new works performed by the Center for Contemporary Opera, Music Theater Group, Encompass New Opera theater and American Opera Project, organizations all in the business of fostering new works.

The entire program kicks off tomorrow at noon with a panel discussion, moderated by Mark Adamo, the composer of "Little Women," titled "Transforming Literature Into Opera." But the pertinent talk usually takes place in the lobby among audience members, both insiders and buffs, who compare impressions of the new works.