Is there a pronunciation rule for unstressed я?
I remember being told that it was closest to English i, for example, as in десять but this doesn't always hold. It seems that at the end of a word it can be either a or ya: бояться, сегодня.
At the beginning of a word: являться is it yi, i or ya? There is clearly a disagreement between this and this speaker.
In standard pronunciation, unstressed я is reduced to /ɪ/. See, for example, this Wikipedia article for a good explanation of the subject.
However, one thing that should be kept in mind is that letters е, ё, ю, я mean different things after different letters:
After consonants, they are not yotized and denote /э/, /о/, /у/, /а/ (not taking reduction into account, here); in addition, the preceding consonant is palatalized (becomes soft): десять /'dʲesʲɪtʲ/.
In other cases (after vowels, soft and hard sign, and in the beginning of a word), the vowel is yotized: ясный ‘clear’ /'jasnɨj/.
In your case, it's yotized and reduced, so, based on my own (hopefully pretty standard) pronunciation:
являться /yɪ'vlʲat͡sa/
In Moscow speech (which is the base for the standard pronunciation used on TV broadcasts and such), the initial unstressed я would be yotized and reduced to ɪ (near-close), something between е and и.
Some dialects (mostly Northern) reduce it further to i (close), like the sound denoted by и. This is OK in most words and would probably not be perceived at all by the majority of the audience, but could result in a hiatus in words like яичница which is considered undesirable in Russian speech.
Some dialects to the south of Moscow exhibit yakanye (яканье). Those dialects would only slightly reduce the sound to a near-open æ or even won't reduce it at all (leave an a). This is considered dialectal.
There is a rhyme to mock a person who uses yakanye:
А у нас в Рязани
Грибы с глазами
Их едят (ядять),
А они глядят (глядять)!
When telling this rhyme, vowels should not be reduced at all, and the word едят should start with a yotized a (as open as possible). Also, to mock the other aspects of the dialect, Г should be pronounced fricative (ɣ rather than g), and the final consonants т should be softened (palatalized).
The pronunciation of я depends on whether it is preceded a consonant or not. Generally, it is pronounced as [ja] [йа], however if it is preceeded by a consonant it softens the consonant and is pronounced as [a]. For example, in бояться the first я is pronounced as [йа], because it is preceded by a vowel, but the second я is pronounced as [a] (and the syllable is pronounced like сьа), because it is preceded by a consonant.
