The “m” is pronounced “em,” so the word an should be used, correct?
correct.
I have always heard it ‘a mp3’ but I don’t know if that is correct grammar or not, or if it is a regional Texas thing.
It’s “a” or “an” based on what the initial vocalized phoneme (sound) is – consonant or vowel. In this case, it’s spoken as “Emm Pee Three” and takes “an.” (This of course requires one to decide if an acronym is to be uttered as a word or as its constituent letters – which is generally but not always clear.)
I decided to go with one of each. 
Thanks everyone.
I’ve always said an mp3. It’s good to know that’s correct.
I have this same problem with the word “FAQ” (Frequently Asked Questions). If you pronounced it like ‘fak’ it should be a FAQ, but if you pronouce the letters out, ‘F-A-Q’ it should be an FAQ. That’s what I always write, but if someone else reads the word and is sounding out fak in their head, it looks to them as if I have bad grammar.
I agree with Eureka.
Obviously, it’s “a mupthry”.
The coward’s way out.
“An” is correct; “A” is not.
A classic example of this (at least in my line of work) is SQL which is pronounced either ESS-CUE-EL or SEE-kwel and occasionally SKWEEL. So writers just go with “a” or “an” depending on how they say it.
It is purely a phonetic question. One of my pet peeves is hearing, “An historical…” from someone pronouncing the “h”. Then you see an occasional non-native speaker writing, “An uniiversity…”, because they have learned a rule, but not in the correct one (or else they don’t know how “university” or other “u”-words are pronounced.
Right on. Two vowels in a row are hard for us to pronounce. When a word starts with a vowel sound we use “an” and otherwise we use “a.” mp3 begins with a vowel sound.
Either way meets both the usage and understandability tests. Both are correct.
This brings up an interesting thought. This rule depends on “A” ending in a vowel sound (“Ah” or “Uh” pronunciation). But I imagine there are accents/dialects where “A” is pronounced “Ay” (long A) such that it ends in a more consonantal “y” sound. In this case, “A” could always be used and “An” never needed. Are there English speaking regions where this is the case?
i don’t know whether the "ay"pronunciation helps.
With “ay ant” you still come to a stop after “ay.” I guess maybe you could run the “y” into the next word, “ay yant.”
MP3 is an abbreviation, not an acronym. An abbreviation uses the letters as they are pronounced (e.g. “em pee three”), whereas an acronym uses the word spelled by the letters (e.g. Nasa, Laser).
“Em” therefore takes an “an”.
I’m usual on the liberal/anti-prescriptivist side of most language debates, but in this case, I cannot think of any good reason to use “a mp3.” I don’t think there is a single stylebook in the English language that would say it’s correct. Of course “a mp3” is understandable, as is “a elephant,” but I don’t think either are correct by the rules of Standard American English. You’d have to provide a cite, therefore, to claim that both are correct.
There are no “rules of Standard American English”. There are various stylebooks, some are prescriptive and some are descriptive. The descriptive ones would all agree with me- if the use is common and it is understandable, then it’s OK. Since I do not work for any companies that require the use of any of the prescriptive books, I don’t give a damn what they say. You might prefer one of the various descriptive stylebooks and that’s perfectly OK. You can then say “according to the Chicago Manual of Style…”. But you can’t say according to “rules of Standard American English” since there is no such thing.
I found that “a mp3” gave me 1,870,000 hits. Thus, the usage is common. The usage included several site devoted to grammar but none where “a mp3” was used as an example. “An Mp3” revealed 1,170,000 hits, so the usage is slightly less common but still common enough.