Troyanos/Domingo on PBS

Tonight most PBS stations kick off a long fund-raising drive with a ''Live From the Met'' gala concert featuring Tatiana Troyanos and Placido Domingo, with James Levine conducting the Met orchestra in a performance taped at the house Sunday evening.

The program was conventional, the singers were in moderately good voice, and voices always sound rounder, fuller on these PBS telecasts.

Domingo certainly did not run into the grievous problems he evinced on the ''Live From Studio 8-H'' last month. There were phrases here and there that caught the special quality of line and shaping of phrase that used to mark so much of his work.

Miss Troyanos seemed more at ease in the first half of the program. The second half found her forcing for notes and effects, and with the orchestra up on stage, both singers had their share of inaudible moments in the house. It all made for a pleasant outing at the Met and should make for pleasant simulcast listening, though nothing about it smacked of the gala it was billed as, at up to $100 a ticket. On most PBS stations, tonight's broadcast will be repeated (check local listings).