I've always heard it pronounced /bɒld/ (rhymes with scald, for those of you who don't know IPA), however the dictionary and some of my friends say /bɔ:ld/ (rhymes with mauled). I'm British, by the way. Any insights? Please say where you're from with your answer.
The pronunciation of "bald" as /bɔ:ld/ is older. What seems to have happened historically for some speakers of British English is phonemic shortening of the sound /ɔ:/ to /ɒ/ in some cases when it comes before a consonant cluster /lC/ (where C stands for any consonant). This change, and the resulting variation in pronunciation, is described in the following post on John Wells's phonetic blog: scolding water. Here is the relevant paragraph:
It is well known that words like salt can have either ɔː or ɒ. Although I have continued to prioritize ɔː in LPD I have to confess that the most recent poll I did on salt showed a sharp trend of change over time in the direction of ɒ. The youngest age group voted 71% for the LOT vowel. Only 34% of my own age group voted that way, which accounts for my bias in favour of THOUGHT in this and similar words (i.e. words in which the vowel is followed by l and a voiceless consonant — alter, false, fault, waltz etc.). Where the consonant after the lateral is voiced I believe there is less variation. Nevertheless I recall that my mother pronounced scald as skɒld, which was odd because she had the expected ɔː in bald, alder etc. (Why, even as a child, did I think her pronunciation of scald was odd?)
I also found this Livejournal post about it: scolding slurry, which suggests that it is connected to the loss of the LOT-CLOTH split in most modern forms of standard British English. Apparently, another environment where shortening occured was in words like austere (listed as having the CLOTH vowel /ɒ/ by the British Library). This shortening is further discussed in this article by Piotr Gąsiorowski: The History of [ɔː]: Is There Regular Orthographically Conditioned Sound Change?; Gąsiorowski mentions a few more words with exceptional /ɒ/ such as sausage, laurel and cauliflower. I'm not sure if any American dialects that maintain a distinction between the vowels of COT and CAUGHT show the effects of such shortening in salt and related words; it seems to be mainly a British phenomenon.
It appears that your dialect shows shortening of /ɔ:l/ before voiced consonants as well, such as /d/. But this does not apply before the secondary cluster /ld/ formed when verbs that end in /ɔ:l/ are followed by the past suffix /d/. (There are attested sound changes with conditions like this in some other dialects of English, such as the Scottish vowel length rule or the distinction between words like freeze/frees and bruise/brews in Geordie.)
So for you not only halt, vault, false, alternative /hɒlt/ /vɒlt/ /fɒls/ /ɒlt.../, but also bald, scald /bɒld/ /scɒld/ have the same vowel as golf, doll, dolled /gɒlf/ /dɒl/ /dɒld/. But walk, maul, mauled /wɔ:k/ /mɔ:l/ /mɔ:ld/ have the same /ɔ:/ as torn, sort: they were not shortened because the first one lacks a pronounced /l/, the second one lacks a consonant after the /l/, and the third one has a consonant cluster only across morpheme boundaries.
The Oxford English dictionary (OED) lists alternative pronunciations with /ɒl/ instead of /ɔ:l/ for halt, vault, false, fault; but not for bald and scald. This fits with what Wells says about most speakers only having shortening before clusters of l + a voiceless consonant. However, there also seems to be some word-by-word variation, as in the case of Wells's mother.
Unfortunately, there's not a huge amount of evidence for the outcome of words with /ɔ:l/ followed by a voiced consonant, because I can't find any other common words that end with auld/ald. But here are some test words with this sound in the middle of a word; I'd be interested in knowing how you (or any other British English speakers) pronounce them:
- cauldron (OED only lists /ɔ:l/)
- alder/alderman/alderwoman (OED lists /ɔ:l/ and /ɒl/)
- baldric (OED only lists /ɔ:l/)
- thraldom (OED only lists /ɔ:l/; I'd assume the morpheme juncture means no one uses /ɒl/)
This analysis is just based on your described pronunciations. I have no personal experience with this; I'm American with the COT-CAUGHT merger, so I have /ɑ/ in all of these words.