Written by Dyami Millarson
The unstressed Old Norse vowels i and u are spelled as e and o respectively in manuscripts (abbreviated as MSS.). Henry Sweet wrote in 1886 in London: “The unaccented i in systir, etc. (which is generally written e in the MSS.) probably had the sound of y in pity, which is really between i and e. The unacc. u in fōru (they went), etc. (which is generally written o in the MSS.) probably had the sound of oo in good.”
Unaccented means here that the described vowels only occur in unstressed syllables.
The sound of y in pity is a close front vowel in any case, but establishing its phonetic quality requires special attention. There is a historical phonetic relationship between [i] and [ɪ] in the English language: happy tensing, which is a phonetic process whereby the tense vowel [i] is moved to the lax vowel [ɪ], has already occurred in the English language of the Southern British Isles by the late 1800s, which is the geographic and temporal context from which Henry Sweet writes. We must thus assume that Henry Sweet means pity to be pronounced /ˈpɪtɪ/ rather than /ˈpɪti/. In other words, he means that unaccented Old Norse i and u are to be phonetically represented as [ɪ] and [ʊ].
Some phonologists are reluctant to specify the quality of the y in pity as they treat the y of pity to be an archiphoneme (underspecified sound) that is properly neither [i] nor [ɪ].
If unaccented [ɪ] and [ʊ] were written as e and o in Old Norse, it would make sense because the aforementioned vowels are still used to represent the exact same sound qualities in other languages, such as English and Swedish and Dutch, to this day. For instance, Swedish still uses o to represent [ʊ], the double oo in Dutch may be pronounced as [ʊː] by some speakers, and the double oo of foot may be pronounced as [ʊ] by Britons. If we are to interpret the spelling choice of the Old Norse manuscript writers, we may note the Old Norse accented /e/ and /o/ would have sounded very much like the unaccented /i/ and /u/. While the Old Norse manuscript spellings suggest a close phonetic relationship between accented and unaccented /e/ and /o/, the vowel qualities [ɪ] and [ʊ] make sense as interpretations for the sounds of unaccented /i/ and /u/ which were spelled as /e/ and /o/ respectively in the manuacripts. It makes sense to desire to find a phonetic explanation for why the Old Norse unaccented i and u are spelled e and o, and I commend Henry Sweet for bringing this issue up, as it deserves being pondered on.
The reduction of Old Norse vowels /i/ and /u/ in unstressed positions makes sense as it is easy to pronounce lax instead of tense vowels in unstressed positions; I already had a spontaneous tendency to pronounce the /i/ and /u/ laxly rather than tensely in those positions. The Old Norse /i/ and /u/, whether stressed or unstressed, have been generalised to [ɪ] and [ʊ] in Faroese, which are the lax equivalents of the tense vowels [i] and [u].
I should clarify, some Faroese speakers pronounce these vowels in all positions, but others pronounce reduced vowels in unstressed positions. So, some Faroese speakers may say [ø ~ ə] for [ɪ] or [ʊ] when the vowel is positioned in an unstressed syllable. It is particularly interesting that the phenomenon of vowel reduction specifically applies to the phonemes /i/ and /u/ in Faroese while it may also be expected to have specifically applied to the same phonemes in Old Norse; the occurrence of this phonetic phenomenon in a descendant is further confirmation that vowel reduction may have been a real thing in Old Norse in the sense that it is not really that strange to think so while Faroese shows how such a thing may work.