The question is: "What sound difference
there is between a long a as in calm and a short o as in top?"
So we're looking for the difference
between the pronunciations, if any, of calm and top as that would seem to be the pronunciation that the text in question is using.
The problem is that the U.S. pronunciation is varied and the RP isn't. We've run up against this before.
Merriam-Webster online dictionary gives pronunciations that we can use.
http://www.merriam-webster.com/cgi-bin/ ... ary&va=top
http://www.merriam-webster.com/cgi-bin/ ... ry&va=calm
Pronunciation: 'käm, 'kälm, 'kam, 'ko(l)m, New England also 'k[a']m
The second pronunciation clip of calm ['ko(l)m] has the difference we're looking for.
The Merriam-Webster Pronunciation Guide clarifies the difference:
(Explanation of \ ä \)
as in bother, cot, and, with most American speakers, father, cart. The symbol \ä\ represents the vowel of cot, cod, and the stressed vowel of collar in the speech of those who pronounce this vowel differently from the vowel in caught, cawed, and caller, represented by \o\. In U.S. speech \ä\ is pronounced with little or no rounding of the lips, and it is fairly long in duration, especially before voiced consonants. In southern England \ä\ is usually accompanied by some lip rounding and is relatively short in duration. The vowel \o\ generally has appreciable lip rounding. Some U.S. speakers (a perhaps growing minority) do not distinguish between cot--caught, cod--cawed, and collar--caller, usually because they lack or have less lip rounding in the words transcribed with \o\. Though the symbols \ä\ and \o\ are used throughout this dictionary to distinguish the members of the above pairs and similar words, the speakers who rhyme these pairs will automatically reproduce a sound that is consistent with their own speech. In words such as card and cart most U.S. speakers have a sequence of sounds that we transcribe as \är\. Most speakers who do not pronounce \r\ before another consonant or a pause, however, do not rhyme card with either cod or cawed and do not rhyme cart with either cot or caught. The pronunciation of card and cart by such speakers, although not shown in this dictionary, would be transcribed as \'k[a']d\ and \'k[a']t\. Speakers of r-dropping dialects will automatically substitute \[a']\ for the transcribed \är\. (See the sections on \[a']\ and \r\.)