Skaters in the dance events at the United States Roller Skating Championships didn't have to bother with recorded music for their routines. Organist Dominic Cangelosi took care of that. Since 1969, Mr. Cangelosi, of Burbank, Calif., has expertly brought the sounds of waltzes, tangos, fox trots, and other dance music to the United States Roller Skating Championships. He is one of about 20 organists in the US who provide live music for roller-skating competitions. ''It just makes the meet more exciting,'' says Cangelosi. He practices roller-skating music's pulsing harmonies (which often sound as though they are slightly submerged underwater) for a few hours each day. Cangelosi chooses his own music for the compulsory dance events, making sure the music conforms to tempo and rhythm requirements. His repertoire includes classical music, show tunes, and even contemporary material. (One number, for example, sounded like an adapted Boyz II Men single.) ''The timing is crucial,'' Cangelosi says, referring to the strict tempo he must maintain for skaters. Ticking the beat for him at the championships was a metronome built in to the computerized Yamaha organ. Cangelosi must also keep each dance number to exactly three minutes, which he does by using a stopwatch. Also important, he says, is the ability to improvise. Cangelosi got his start in organ playing when he roller-skated as a youth and noticed the organist at the rink. Roller-skating lessons segued into organ lessons, and before long he was taking jobs playing the organ. Professional skaters, he says, coached him on the fine points of roller dance music. Cangelosi now owns his own roller rink in California, and he also records roller-skating music as part of Rinx Records.