I was reminded while teaching this week how tricky the word audio is, but how instructive it can be. When non-natives pronounce this word in English, they can be misled by the spellings of the first and last vowels. The spelling au suggests a complex vowel that changes in quality (a diphthong); the spelling o suggests a simple, pure vowel that doesn’t change (a monophthong). But in English the reverse is true.

The vowel at the beginning of audio is a pure monophthong, known as the THOUGHT vowel because it’s the vowel in the word thought. This vowel is most commonly shown in dictionary transcription as /ɔː/ (British and ‘Eastern’ American) or as /ɑː/ (‘Western’ American). These symbols indicate long vowels which don’t change in quality. Here I am saying BrE audio, normally then with a prolonged first vowel to demonstrate how unchanging it is:

The vowel at the end of audio is a diphthong, the GOAT vowel (yes, it’s the vowel in the word goat). It’s most commonly shown in dictionary transcription as /əʊ/ (British) and /oʊ/ (American). These symbols indicate the different qualities at the start and end of the vowel. Here I am saying BrE /əʊ/, first at normal speed, then prolonged to reveal the change in quality from an ə-type sound to an u-type sound:

Here are some transcriptions of the word audio from online dictionaries. They’re all different, but at least they all agree that the first vowel is a monophthong, and the last vowel is a diphthong:

British: /ˈɔː.di.əʊ/ (Cambridge), /ˈɔːdɪˌəʊ/ (Collins), /ˈɔːdɪəʊ/ (Oxford Dictionaries)
American: /ˈɑː.di.oʊ/ (Cambridge), /ˈɑːdiˌoʊ/ (Merriam-Webster Learner’s), /ˈɔːdioʊ/ (Oxford Learner’s)

Further notes

The word audio is actually quite consistent with generalizations of spelling and pronunciation in English. Hundreds of words end with the final letter o pronounced as the GOAT diphthong:

hello, no, so, also, go, ago, euro, hero, photo, solo, studio, video, Romeo, Chicago, Toronto, etc

but final o is never pronounced with the THOUGHT monophthong. On the other hand, au is pronounced in hundreds of words as the THOUGHT monophthong:

August, author, auto, autumn, cause, fraud, haunt, launch, laundry, Paul, pause, etc

but au is not pronounced with the GOAT diphthong – except in French borrowings like:

aubergine, au pair, au fait, chauffeur, faux pas, Flaubert, gauche, mauve, sauté, Sauvignon, etc.

The week began with major figures in Britain’s Labour Party refusing to serve in the shadow cabinet of Jeremy Corbyn, their new left-wing leader. London’s Evening Standard newspaper told us that

Chuka Umunna stood down as shadow business secretary complaining the new leader had refused to give him an “wholehearted” assurance he would not campaign to stay in Europe.

An wholehearted assurance? I think many native speakers would agree that an looks a bit odd here and feels even odder to say. The indefinite article is generally a before a consonant (eg a child) and an before a vowel (eg an adult), and whole begins with the consonant /h/.

We do sometimes find an used before h in adjectives like historic, hilarious and hysterical, and nouns like historian, horizon, and hallucination. This is related to stress. English /h/ is /h/appiest in stressed syllables. By contrast, in unstressed little words like he, her, him, /h/ is often weakly pronounced or dropped altogether – even in careful speech – so that we hear eg What did you /gɪv‿ɪm/ (give him)?

The first syllable of words like historic is similarly unstressed, and the weakness of the /h/ there encourages the use, by some at least, of preceding an rather than a. Personally, I find it a bit more comfortable to drop the /h/ and say /ən‿ɪ/storic event

than /ə‿hɪ/storic event

but the latter is perfectly standard contemporary English.

So what about wholehearted? Its main stress is on the middle syllable. If I type ‘wholehearted’ into Google, I’m presented with this:
which suggests that the first syllable whole- is unstressed. But whole- is not fully unstressed; many dictionaries give it a secondary stress mark, eg /ˌhəʊlˈhɑː.tɪd/. Furthermore, in a phrase like wholehearted assurance, whole- may well be more prominent than -hearted (thanks to the phenomenon known by names such as ‘the rhythm rule’ or ‘stress shift’):

which makes the use of an before it seem rather odd. The simplest recommendation for non-natives, I think, is always to use a, not an, before the sound /h/ (as in happy, historic and wholehearted but not in hour or honest, where the letter h is silent). This is always correct.

Further notes

To some extent the use of an before /h/ is a/an historic hangover. It seems that a century or more ago /h/ was never pronounced in unstressed syllables, so the h in historic was as silent as the h in hour or honest. Gradually such h‘s began to be pronounced.

A few years back John Wells’s blog discussed the similar case of an homosexual.

In my experience, non-natives often mispronounce the word national. They wrongly assume that the first part is pronounced the same as the word nation.

In native pronunciation, however, nation has the long a of face, day and major, while national has the short a of trap, fashion and passion. Here are the two words, each followed by its first part:

(In the transcription of Oxford Dictionaries, these are /ˈneɪʃ(ə)n/ and /ˈnaʃ(ə)n(ə)l/. The parentheses indicate that the weak schwa sounds may not be sounded at all.)

So nation rhymes with station, location and situation (in fact with all words that have the suffix -ation), while national rhymes with rational, irrational – and not much else. You might say it’s quite an irrational pronunciation.

Note that ration also has short a. There are only three common 2-syllable words ending -ation, namely nation, station, ration, and ration is the odd one out: /ˈraʃ(ə)n/.

Further notes

As Eric Armstrong (@voiceguy on Twitter) points out, American English allows ration to be pronounced with the short a or, alternatively, with the long a like nation and station. The short variant /ˈræʃən/ is the first or only variant given in the American dictionaries I’ve consulted.