Re: The a sounds in “arable”, “arid”, “apparel” ...
In all three words, the stressed syllables are: ar, ar, par. Given the nature of "r", the low vowel /ae/ tends to become a bit higher, becoming /ɛ/. The high vowels like /i/ and /u/ tend to become lower before r: /ɪ/ and /ʊ/. This is called pre-r neutralization.
Both m-w.com and learnersdictionary.com lists the same sounds (in their mp3s), but the latter dictionary transcribes with a higher vowel in all three words. When your hear the sound, in the case of 'arable', I hear /ɛ/. I hear /ae/ for other two words. This is a case of "applying some or another phonological rule in transcription, but not applying it in recording".
Anyway, to makes things not complicated, arV when stressed usually become /ɛɚ/. arC becomes /ɑɚ/