Joni Mitchell, one of rock music's most exquisitely voiced performers, sang at Constitution Hall last evening before a house sold out many weeks in advance.

Most in attendance were between the ages of 16 and 25 and the audience greeted each new song with unusual warmth and vigor.

She sang in a crystalline voice that covered a staggering range: from deep-throated invocations of the soul to incredibly high floating tones that almost seemed to provide their own harmonies.

But besides her voice, she's also gifted with a most lyrical ability to write. Her songs are strongly personal in substance, ranging from sentimental laments for the hip sub-cultures of Woodstock and California to memories of old lovers to transformations of everyday experiences such as finding a musician on the street singing for free.

In addition, she displayed thoroughly adept ability as a performer, swaying hypnotically as she strummed her guitar and talking congenially with the audience about her experiences on a trip to Greece, for example.

Offstage, however, she becomes quite shy. Her road managers are protective, insisting that she gives no interviews and will not even speak with anyone from the press.

"Too much misquoting used to happen and too many angles were being taken," one said yesterday when asked about the possibility of an interview.

But on Tuesday afternoon a reporter casually met her as she was walking along M Street in Georgetown, taking pictures with a camera she said she'd received as a Christmas present. Asked if she would consent to a brief interview, she somewhat sheepishly replied that she didn't think she had time. Then with a grin she hesitantly said, "I really don't like interviews too much."

She chatted for about 10 minutes. When asked why she had decided to return to the road after a two-year absence from any concerts, she responded: "Well, I wrote a lot of new songs and I wanted to sing them for people." She sang several of them last night and they sounded just as good as her well-known material.