AFAA, in the Fitness, Theory & Practice book recommends that the music in seated aerobics classes be faster than used in studio aerobics (up to 170 beats per minute). The reasoning that they give for this fast music is that the limited muscle mass above waist level requires additional stimulation to achieve a training heart rate. Disabled Sports USA, in the Fitness Programming and Physical Disability book, recommends tempos slower than 150 beats per minute to allow full range of motion in the exercise.
In practice, I have found that music a little slower than the normal low-impact tempo (140 +/-) and somewhat higher than the normal step temp (124 +/-) works for my seated classes. Music at this tempo is usually referred to by the mixing companies as Mid-Tempo Music (Power Productions Dynamix and Muscle Mixes all have tapes in this tempo). It is usually marketed as slide or aqua music, so you may already have some in your tape box. I look for tapes with a tempo of about 135 beats per minute. As with aerobics and step, once you have taught using mixed tapes, you won't try homemade again.
By the way, don't be afraid to use music differing from what I tell you; you have to assess the fitness level of your classes and set the tempo accordingly.