As a native speaker, I intuited that the choice of a or an depends on the sound of the following letter not whether it is a vowel (the word you were looking for) or not.
A google search confirmed that my intuition is considered correct.
But my intuition gets me in trouble with "h" ... I say "an historical ...," but others say "a historical." And Google says there are differing opinions on that one.
So, with regard to the word ‘university’, the initial sound is actually the consonant sound /j/, so we say ‘a university’. If we look at your original question, the word ‘LP’ begins with a vowel sound, /e/. That is why we say ‘an LP’. Source: bbc.co.uk http://www.bbc.co.uk/worldservice/learningenglish/language/askaboutenglish/2010/06/100622_aae_indefinite_articles_pron.shtml