What happened to voiced velar fricative [ɣ] and velar approximant [ɰ] in English language?
The voiced velar fricative is a type of consonantal sound, used in various spoken languages.
Wikipedia says that it is not found in English today, but did exist in Old English.1
Why did this sound disappear? What happened in the history?
Some other Germanic languages still have that. Might it still exist in some dialects or casual speech in English?
Some relevant details1:
The symbol ɣ is also sometimes used to represent the velar approximant, though that is more accurately written with the lowering diacritic: [ɣ̞] or [ɣ˕]. The IPA also provides a dedicated symbol for a velar approximant, [ɰ], though there can be stylistic reasons to not use it in phonetic transcription
Examples from other languages1:
1 http://en.wikipedia.org/wiki/Voiced_velar_fricative
In Old English, /ɣ/ is a phoneme, but the [ɣ] allophone occurred only in a rather specific condition: when word-medially, though not when in gemination, directly after nasals, or directly before a front unrounded vowel or palatal glide.
The word dagas ‘days’ (nom./acc. plural), for example, would have been pronounced something like [dɑɣɑs]. fugol ‘bird’ (nom. singular) would have been something like [fuɣol].
OE [ɣ] descends from Proto-Germanic /ɣ/, which had a much more widespread distribution. See Richard Hogg's (1992) Grammar of Old English, volume 1, §2.56, for more detail and references.
In Middle English, [ɣ] became vocalized. If it followed a front vowel, it became an instance of /j/, and if it followed a back vowel, it became an instance of /w/. In both cases it went on to form new diphthongs, as in fowl, from fugol. See p117 of this book. At that point there is no longer a /ɣ/ phoneme, or even any allophones (though some do develop later in varieties of English, as David Garner points out).
The voiced velar fricative /ɣ/ does exist in Liverpool speech, but I'm fairly sure it's a more recent development of intervocalic /g/. I haven't managed to figure out exactly when /g/ does and doesn't change to /ɣ/, but here are two examples. 1. The town-name Maghull [stressed on the second syllable] is pronounced /məˈɣʊɫ/. 2. If you watch just about any episode of Red Dwarf [UK version], you'll hear Dave Lister call Arnold Rimmer a smeghead /ˈsmɛɣɛd/.