I came across the following excellent description of the old vowel-
system by Guðmundur Sæmundsson. If you want to know a little about
how Old Norse was pronounced, then please read this:

Í fornmálinu var tvennt gjörólíkt nútímamáli:

In the old language was two/a pair (2 things) totally-unalike the
modern language (now-time-language):

1. Í fyrsta lagi: Löng sérhljóð og stutt höfðu merkingargreinandi
hlutverk. Í orðinu 'of' var o-ið stutt, en í orðinu 'óf' var o-ið
langt. Það var eini munurinn á þessum tveimur orðum, og þess vegna
var það merkingargreinandi. Þetta er ekki svona í nútímamáli. Við
höfum breytt sjálfu hljóðinu í seinna orðinu til að ná fram merking-
armun. Þetta höfum við gert við flest gömlu hljóðin, þ.e.a.s. þau
sem voru skrifuð með broddi fyrir ofan.

1. In the first place: Long vowels and short had a meaning-defining
role (to play). In the word 'of' was the O short, but in the word
'óf' was the O long. That was the one difference on (=between) these
two words, and on account of this was that (=the difference) meaning-
defining. This is not such in the moderen language (now-living-lang-
uage). We have changed self the sound (=the sound itself) in the
later word to bring forth the meaning-difference (=the difference in
meaning). This have we done with most old (vowel) sounds, that is to
say - those which were written with (an) accent on top.

2. En þetta er ekki það eina. Gömlu löngu hljóðin gátu líka verið
nefkveðin, og þessi nefjun gat verið merkingargreinandi. Nefjunin
kom einkum fram þar sem nefhljóð voru á eftir viðkomandi hljóði, eða
höfðu verið það (=þar), en voru nú horfin með einhvers konar brott-
falli. Dæmi: 'frá' gat þýtt tvennt, forsetningin 'frá' sem áður
hafði verið með n-hljóði (*fran) og lýsingarorðið 'frá', sbr. 'frá á
fæti', en í því hafði ekki verið neitt nefhljóð. Hið fyrra var því
borið fram nefkveðið, (fran:), en hitt ekki, (fra:). Nefjun kom
aldrei fram í stafsetningu fornritanna, nema hjá fyrsta málfræðing-
num sem táknaði þau með punkti fyrir ofan. Við táknum þau með litlu
upphækkuðu n-i.

2. And this is not that one (=the only thing). The old (vowel)sounds
could likewise be nasalized (=nose-quothed), and this nasalization (=
nose-quothing) could be meaning-defining. The nasalization came
especially forward there where nasal-(vowel)sounds were on after (=
followed) the with-coming (=pertaining) (vowel)sound, or had been
there, but were now gone with (=by,through) some kind of away-falling
(=syncope). Examples: 'frá' could mean two/a pair, the preposition
'frá' which before had been with n-sound (*fran) and the adjective
'frá', compare 'frá á fæti', but in this (the latter) had not been
any nasal-sound. The former was thus (=of that) pronounced (=bared-
forth) nasalized (nose-quothed), (fran:), but the other (the latter)
not, (fra:). Nasalization came never (=of age not) forth in spelling
(stave-setting) of the old writings, except with the first grammar-
ian (=the anonymous author of an old grammatical treatise) who drew
(represented) them (=the nasalized vowel-sounds) with a point (dot)
on top. We (will) draw (represent) them with (a) little up-hiked (=
raised) n.

(note: I will put the 'little raised n' in paranthesis instead in
what is to follow - see my next post on this topic)