In the above cases,
SOS is pronounced "es o es" but not "sos"...
Similarly, MP is pronouced "em pee" but not "mp"
The difference between "a" and "an" depends on vowels but also for certain alphabets such as:~
A, E, F, H, I, L, M, N, O, R, S, U...
p.s. this is not a definite rule but simply some common usage.