I thought that French has only /ɔ/ sound, which is a bit more relaxed sound then /o/sound. But when I try to find the pronunciation symbols for my vocabulary book, both of the rounded vowels appear.
For example, the words 'comme' and 'alors' come with /ɔ/sound. 'Cause' in 'a cause de' appears with /o/.
Do I have to memorize these different pronunciations? Surely there must be some rules as to how to pronounce it, mustn't it?
I wouldn't worry too much about the distinction between /ɔ/ and /o/. In French French, /ɔ/ and /o/ tend to merge to /o/ in non-final open syllables, contrary to varieties spoken in Belgium or Quebec. There are only few words that are distinguished with these sounds to produce a difference in meaning. I'm thinking right off the bat of comme /kɔm/ and Côme /kom/, bosse /bɔs/ and Beauce /bos/. For other examples of minimal pairs, see Eauquidort's comment. Mistaking the two or blurring them will not, I believe, impair understanding and communication much.
Now there are other sounds that need to be clearly separated in pronunciation because very often they make it possible to distinguish high-frequency words or meaningful units between each other, such as, for example /ɔ̃/, /ɑ̃/ and /ɛ̃/. Think of qu'on, con /kɔ̃/, quand, qu'en /kɑ̃/, qu'un /kɛ̃/, bon /ɔ̃/, banc /bɑ̃/, bain, ben /bɛ̃/ etc. The same would be true of /y/ and /u/ in tout and tu, vous, vu, roue, rue etc.
This is the case in English as well. Somebody that would mistake /i:/ for /ɪ/ or have no distinction between the two, which is typical of a strong French accent, and pronounce leave and live, fit and feet, did and deed, Pete and pit, read and rid or seat and sit in the same way with French /i/ would make it harder to be understood because the opposition between /ɪ/ and /i:/ is so important in English.
A contrast between close-mid /o/ and open-mid /ɔ/ is present in many varieties of French. But the distribution of the two sounds varies between accents. Some accents have distinctions in vowel length as well as in vowel quality, but I think that most French learners are not taught an accent with vowel length distinctions.
In some positions, the contrast is "neutralized", and you can predict with confidence that either [o] or [ɔ] will be used. For example, many speakers in France can only have [o] when the vowel sound is at the end of a word, as in pot and peau (both pronounced [po]). But in some French-speaking regions, /ɔ/ is possible at the end of a word.
In the north of France, either /o/ or /ɔ/ can occur before a word-final consonant sound. As LPH's answer says, many spelling patterns are associated with one sound or the other. The sound [ɔ] mainly appears in words spelled with the uncircumflexed letter o. The spellings au, eau, ô tend to indicate [o], as in saute [sot]. A plain letter o can also be pronounced as [o]. Before a word-final [z] sound, [o] tends to be used, as in rose [roz].
A well-known feature of accents in the south of France is the use of [ɔ] in place of [o] in syllables ending in a consonant sound. A speaker from southern France might use [ɔ] in words like saute and rose (you can see a map on this website: "Petit guide linguistique à l’usage des gens du Nord en vacances dans le Sud de la France", par Mathieu Avanzi, Français de nos régions).
There's no rule, but there are patterns. For example, words with the same syllables tend to have the same pronunciation.
école, coupole, créole, agricole, vole are all /ɔ/
austral, hausse, cause, exhausser, fausse, gaussien are all /o/
binôme, biome, lymphome, dôme, diplome, atome are all /o/
comme, homme, consomme, dégomme, nomme, pomme, gomme are all /ɔ/
Of course there are exceptions, but if you're unsure you can try that and most of the times you won't be too far off.
However, you should pay attention to where the syllable is in the word, it may be important! With -omm- for example, it's an open "o" only when it's the last syllable. If not, it's a closed "o".
commère, hommage, nommer, consommer are all /o/
voler, abolir, absolu, cajoler, décoler, hologramme, oligarchie are all /o/
It's actually also a common pattern for a "o" syllable to be closed when it's inside the word and open when it's at the end.
You do have to do that, learn each separately; just as in English, you have to learn them case by case (mostly);
English
ɔ: hall, tall, walk, thought, pause, paw, law, north, …
ɒ lock, pot, sausage, Austria, …
French
- ɔ sorte, homme, comme, tonne, pomme, Paul, …
- o rose, pause, cause, oser, dose, …
There is a bit of regularity though (réf.);
The grapheme "au" has regularly the /o/ pronunciation; the /ɔ/ pronunciation is an exception.
Please do not worry for this phonetic difference between /ɔ/ and /o/! In fact they rely on the same supra phonemic category /O/ and are never opposed in a minimal pair. Whatever your prononciation people will understand you.
a phonetic rule : /ɔ/ can officially not occur at the end of the word (but some local exceptions exist, for example /velɔ/ for bicycle in the region of Besançon.
an orthographic rule: The theory says that when you hear /o/ inside a word it should be written [au] but in some places people do not pronounce differently the two o (for example in Grenoble).
The truly important point is to pronounce differently the French nasal vowels (i.e., [in, un, on, an] =/ɛ̃ œ̃ ɔ̃ ɑ̃/) in the words for example pain, lundi, mon, mange from their correspondant oral vowels pas, /lødi/, mot, mage or you will have some difficulties to be understood.
