[From the October 1999 issue of Rosa, the journal of the Societe des roses du Quebec Rose Society, Fragrance in Roses, by Allen B. Bryant, p. 22:] ... fragrance is due to a complex biochemical process. Briefly, the starch which is made in the leaves from the carbon in carbon dioxide and hydrogen from water is converted to sugar glucose. Glucose is then converted to glucosides of various kinds. The glucosides then decompose into alcohols. Some of these alcohols are oxidized to produce more volatile compounds known as aldehydes. It is these aldehydes that produce the fragrance.
I have found no explanation as to why some varieties produce more aldehydes than others. More red varieties seem to be fragrant than those of other colors. Newly opened blooms are the most fragrant. As the blooms age, fragrance decreases. Double blooms -- those with more than one row of petals -- are usually more fragrant than those with a single row of petals.
Fragrance is more pronounced on warm days, as the chemical processes producing the volatile alcohols and aldehydes are speeded up. Fragrance remains longer in the atmosphere when the humidity is high and there is no wind.
[From The Rose Garden, by William Paul, p. 79:] choose Noisette and Tea-scented Roses for fragrance
[From Botanica's Roses, p. 81: in a discussion of 'Angelique'] There is little in the way of fragrance, as is normal with good cut-flower roses, because the petals of such varieties need to be hard so that the flowers will travel safely to market without bruising; when petals are hard the scent glands function poorly. The myth that roses have lost their scent probably arises from the commercial necessities of the florist trade, although thousands of garden roses have retained their fragrance.
[From The American Horticultural Society Encyclopedia of Gardening, Christopher Brickell, Editor-in-Chief, p. 114:] It is not easy to be specific about rose scent since its character and intensity may vary greatly with the time of day, the humidity, the age of the bloom, and the nose of indidivual gardeners; some will find a particular cultivar fragrant, while others will not.

[From Peter Schneider on Roses, by Peter Schneider, p. 20:] Most mauve roses have a nice fragrance.

1917 The Rose Annual
p54. Rev. Joseph H. Pemberton. Rose Perfumes.