EARLY-MORNING commuters in Boston with a penchant for classical music lost a faithful friend last week. After more than 25 years of offering some of the finest classical programming in the nation beginning at 7 a.m., public radio station WGBH has extended its news broadcasts until 8 a.m. Goodbye Mozart, hello Morning Edition. Farewell Beethoven and Brahms, hello Bosnia and budget cuts. To early birds accustomed to easing their way through rush-hour traffic with symphonies and sonatas, morning drive-time will never be the same. The station defends its changes by claiming listeners ''want more news.'' Although the shifts don't represent a loss of total hours devoted to classical music, the addition of another news program from 4 to 5 p.m. means a similar absence of public-radio music for homeward-bound commuters. Across the country, the hills may be alive with the sound of music, but the question is: What kind, when, and for whom? At station after FM station in city after major city, the threat to classical programming is becoming distressingly familiar. Two years ago WNCN-FM in New York changed overnight from classical music to a ''pure rock'' format, leaving the city with only two classical stations. Several years ago WBUR-FM in Boston, another public radio station, dropped all classical programming. Commercial FM stations in San Francisco and Chicago have faced down similar threats, but everywhere the disquieting concern is: Which classical station is next? Even when classical music remains on the air, other changes have the effect of diminishing its character. Somewhere along the way, announcers turned into ''hosts,'' a title that makes some feel obliged to entertain their ''guests'' with breathless superlatives and egotistical opinions about every selection. To avoid any charge of elitism, some adopt a chirpy, chatty tone, in the vein of ''Aren't-we-having-fun?'' In addition, the selections they play may fall into a narrower, brighter, more familiar repertoire. The formula for wooing listeners of the '90s seems to be: Keep it in a major key. Some culture-watchers see the decline of music education in schools in recent decades as a factor contributing to classical music's endangered-species status among younger generations. Although some FM stations are making worthy attempts to fill the void by offering an hour of ''kids' classics'' on Saturday morning, that can't compensate for music instruction in schools. In another sign of the musically challenged times, the Boston Symphony Orchestra will hold an open house this Sunday to recruit new subscribers. As incentives, the symphony will provide ''beautiful music'' and ''great food supplied by area restaurants.'' As one staff member in the subscription office explains, ''There's definitely a change in ticket buyers. People today want different things. They can't commit to long series, so we need to offer shorter ones.'' That kind of adaptation makes good business sense. But it begs a larger question: Where will classical audiences of the future come from? Will a generation raised on rock eventually acquire a taste for Rachmaninoff and buy season tickets to Symphony Hall? Or will the graying of the classical music crowd continue, with disenfranchised younger listeners never making the switch from MTV to Mahler? To those of us who think a day without Mozart is like a day without sunshine, that's a scary prospect. The availability of classical music - on the radio, in concert halls, in schools - is a cause worth fighting for. Music listeners can understand the shaky bottom-line economics of classical programming and the legitimate desire of radio stations to expand their services - to be more things to more and more people. But, station owners must also understand the concern of listeners, pure listeners, as they stretch their ears toward the vanishing sound of good music.