Dear VS:

You wrote:
> Can I pose this grammar question here?
>
> We all know the rule about a/an. But what is the customary usage in formal
> texts (e.g. a technical reference manual) in the case of acronyms starting
> with a vowell sound? e.g.:
> – a/an FM radio?
> – a/an HTML document?
> – a/an LSD addict?
> – a/an mpg rating?
> – a/an MTV fan?
> – a/an NMR scan?
> – a/an RF connection?
> – a/an SOS signal?
>
> And, by the way, would you recommend hyphenation in (some of these) cases?
> I’d much appreciate an authoritative reply (or a pointer to where I might
> better ask this question).
>
These all would be spoken and written with “an.” All of the first letters when spoken begin with a vowel sound: “an eff-em radio,” “an aitch tee em ell document,” “an ell ess dee addict,” etc. You would, for example, say or write “a DSL connection” (dee ess ell).