Old English had seventeen consonant sounds. All of them were fully pronounced; i.e., there were no "silent letters" even in words like "cniht" (knight). The letters b, d, l, m, n, p, r , t, w, and x were pronounced almost exactly as they are today. The letters þ and ð could represent either the voiced or unvoiced interdental, depending on whether they occurred between voiced sounds such as vowels or not. The same for f and for s [Old English had no letter v or z]--they were voiced when they occurred between vowels and unvoiced otherwise.
The most problematic of the consonants were the c, the g, and the h. The letter "c" could represent either the unvoiced velar stop [k] or the unvoiced alveolopalatal affricate [tē], depending on its location: before consonants and next to back vowels it was always [k]; next to a front vowel (i.e., [i] or [e], it had the affricate sound [tē]. There are certain exceptions to this, based on historical factors. Often the modern word provides a clue to the pronunciation. OE "cepan"--"keep"--had its "c" pronounced as [k] because of the original Germanic pronunciation before Old English.
The "g" was usually written "©" and also had a pronunciation dependent upon its environment. Before consonants and initially before back vowels it had the sound of the voiced velar stop [g]. Initially before front vowels, between two front vowels, and after a vowel at the end of a syllable it had the sound [j] (like the "y" in "yes"). In all other cases it had the sound of the voiced velar fricative [g]. This latter pronunciation became "w" in Modern English. OE "dragan" became ModE "draw."
The "h" at the beginning of a word was pronounced as it is today; in all other positions it had the sound of the voiceless velar fricative [x] (as in the German "Nacht") or the voiceless palatal fricative [ç] (as in the German "nicht"). It was always pronounced, even in combinations such as "hl", "hr", "hn", and "hw".
Old English had six vowels: i, e, æ, a, u, and o. The y was used, but very rarely. In additon, each of the vowels could appear in a diphthong--a vowel combination such as "ie".
The long vowels have the sounds similar to their IPA counterparts, as they would be pronounced in modern Spanish or Italian, for example. All of the long vowels in English were shifted in the Great Vowel Shift of the fifteenth century, so they have different sound in Modern English than they originally had. The "i" was pronounced [i]; the "e", [e]; the "æ", [æ]; the "o", [o]; and the "u", [u].
Short vowels, since they did not go through the Great Vowel Shift, were pronounced much as modern short vowels are.
The tendency to centralize unstressed vowels, so prominent in English from Middle English on, had begun by late Old English, especially in the second vowel of diphthongs ("ie" was pronounced [i´], for example).
The "y" was a rare sound, the rounded high front vowel as in the German "fünf". This sound is lost to Modern English.