Two weeks away from its opening night, with its production of "Carmen" already several weeks into rehearsal, The Washington Opera has substituted Colombian mezzo-soprano Martha Senn for Alicia Nafe' of Argentina, who was originally cast in the title role.

Martin Feinstein, general director of the company, received word last week that Nafe' was suffering from an undiagnosed illness and might not be able to sing again in time for the opening night on Oct. 30. She had recently returned to Hamburg, where she is a member of the opera company, from a tour of Africa, and had contracted an illness (first thought to be a common cold) that German doctors have been unable to diagnose so far.

"I immediately began an intercontinental search, phoning London, Milan, Paris -- all over," Feinstein reports, "and we couldn't find anyone of the caliber we wanted who would be free for this engagement. Then Frank Rizzo remembered that he had auditioned Martha Senn in New York last month for roles in our 1983-84 season. He thought she would be right, so we phoned down to Bogota' and asked her to fly up as soon as possible."

The audition was held late last week in the Kennedy Center Concert Hall, with Feinstein, conductor Rafael Fruhbeck de Burgos, Rizzo (the company's dramaturge) and stage director Michael Kahn -- assisted and perhaps pressured by members of the opera company's staff who had sneaked into the back of the hall to hear the audition.

After Senn had sung most of the role of Carmen for her small audience, Feinstein turned to the others for their opinion but before they could say anything they were interrupted by spontaneous applause from the staff members in the back of the hall. Feinstein glanced at his colleagues and a unanimous decision was reached without any exchange of words.

"If you can be ready at 2 this afternoon," Feinstein told the mezzo-soprano, "you can start rehearsing." She was ready.

Senn was born in Switzerland 28 years ago and raised in Bogota'. For the last three years, she has been studying in New York with Zinka Milanov, Thomas Grubb and Ellen Faull, while also singing leading roles with the Colombian National Opera and other Latin American companies. Her first operatic appearance in this country was in Cincinnati last month for the world premiere of Douglas Smith's "Judith." In the past year, she has won three important competitions: the Baltimore Opera Competition, the American Opera Auditions and the XIV Annual Vocal Competition in Paris.